Kingmak3r’s Most Highly Rated Third Party Sourcebooks

Valda’s Spire of Secrets – Mage Hand Press’ magnum opus. This is a huge rules expansion that adds a ton of innovative new classes and subclasses, plus dozens of new weapons, items, feats, and spells. Some fans seem to consider this a third pillar for 5e next to Xanathar’s and Tasha’s rulebooks.
Griffon’s Saddlebag, Book One – From Hit Point Press, this book contains so many unique items! It seems to be an essential purchase for anyone who wants to enhance the selection of items in their games. It also has a new race, new subclasses, new settings, and even a unique setting agnostic villain!
Griffon’s Saddlebag, Book Two – They made another one! The first one had 300+ items; this one has 500+! Also included are new races, new subclasses, new settings, new creatures. This book is the recipient of a well-deserved Ennie award.
Minsc and Boo’s Journal of Villainy – A sourcebook by the acclaimed James Ohlen (formerly of BioWare) that covers Baldur’s Gate, Athkatla, Suldanessellar, and Ust Natha. It adds new monsters, henchmen, and patrons plus details on factions like the Harpers and Flaming Fist. For Baldur’s Gate II fans who want to know what happened after that game and before Baldur’s Gate III.
Heroes of Baldur’s Gate – Also by James Ohlen. This is a sourcebook on Baldur’s Gate and the surrounding region with new monsters, items, and backgrounds and a selection of the most beloved epic companions from Baldur’s Gate I and II.
Elminster’s Candlekeep Companion – This is a detailed sourcebook on Candlekeep that looks like it would really flesh out the official Candlekeep Mysteries anthology.
Thay: Land of the Red Wizards – A sourcebook on the Red Wizards of Thay by Ed Greenwood, the creator of the Forgotten Realms.
Rashemen: Unapproachable East – Another book by Ed Greenwood and other authors, this is a sourcebook on the wild eastern homeland of Minsc and Dynaheir.
Darkhold: Secrets of the Zhentarim – More from Ed Greenwood and his team, this is a sourcebook about the evil Zhentarim and their stronghold.
The Great Dale: Unapproachable East Campaign Guide – Another sourcebook on the eastern realms, this would go well with the Thay and Rashemen books. A new race, new subclasses, new items, spells, and more.
The Border Kingdoms – Another one by the inimitable Ed Greenwood with details about a chaotic, lawless region in the Forgotten Realms where “anything goes.” Seems similar to Pathfinder’s Stolen Lands.
Book of Ebon Tides – From Kobold Press, this is a sourcebook for the Shadow Realm, a sort of cross between the Feywild and the Shadowfell inhabited by the Shadow Fey. Looks like compelling stuff.
Dungeons of Drakkenheim – Also listed on my other thread, but it didn’t feel right to leave this out. Probably the most highly-rated third party content I’ve ever seen. Designed by the Dungeon Dudes and published by Ghostfire, available for both D&D 5e and Daggerheart. Appears to be heavily inspired by Warhammer’s grimdark Mordheim setting: a cursed city devastated by an eldritch meteor strike. This contains both the campaign setting and a bevy of compelling adventure hooks.
Sebastian Crowe’s Guide to Drakkenheim – From Ghostfire, this book contains additional material for the highly-rated Drakkenheim setting, including a new Apothecary class, new subclasses, spells, and feats.
Monsters of Drakkenheim – This is a Drakkenheim bestiary with over 150 monsters and bosses plus a new crafting system.
Grim Hollow Campaign Guide – Also from Ghostfire, this is a sourcebook for the world of Etharis, a new setting with new spells, feats, curses, backgrounds, and iconic player transformations (Werewolf, Vampire, Lich, etc.). So much lore here.
Grim Hollow Player’s Guide – A rules expansion with new subclasses, races, items, new schools of magic (including Sangromancy – blood magic), character transformations and more.
Grim Hollow: The Monster Grimoire – An excellent sourcebook with 400 new monsters and associated items that can easily fit into any campaign setting.
Grim Hollow: The Raider’s Guide to Valika – A sourcebook on the Nordic-inspired region and culture of the Valikan Clans in the Grim Hollow setting.

Edit: Reddit Community Recommendations:
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Flee, Mortals! The MCDM Monster Book – An enormous bestiary with 300+ monsters, minions and retainers, and “un-boring’d” classic monsters with cool new abilities.
The Monster Overhaul – Another bestiary with useful GM tools for implementing monsters.
Heliana’s Guide to Monster Hunting – Another offering by Hit Point Press, this is a bestiary that also includes new races, items, and spells, as well as rules for harvesting, tracking, crafting and cooking.
Tome of Beasts I, II, and III – A trilogy of bestiaries from Kobold Press with tons of eclectic monsters.
Creature Codex – Another large bestiary from Kobold Press.
Midgard Worldbook – A hefty sourcebook for Kobold Press’ original Midgard campaign setting.
Southlands Worldbook – Another big sourcebook for Kobold Press’ original campaign setting.
Labyrinth Worldbook – A Kobold Press sourcebook for the “paths between worlds,” connective material that could be used to link any campaign settings. This one actually looks pretty intriguing. Could work really well with the official 5e Planescape material.
Vault of Magic – A treasure trove of unique magical items from Kobold Press, including entries by acclaimed fantasy author Patrick Rothfuss, celebrity DM Deborah Ann Woll, and more.
Ptolus: Monte Cook’s City by the Spire – A sourcebook for a sprawling and immersive fantasy metropolis by designer Monte Cook.
Level Up: Advanced 5th Edition – A whole series of rulebooks intended to “level up” your 5e experience.
Tales of the Valiant – A completely revamped version of 5e, essentially its own system with rulebooks, lore, and adventures.
The Dread Thingonomicon – A collection of setting-agnostic locales and NPCs to plug into any campaign.
Tome of Adventure Design – A guide to designing your own adventures.
Spectacular Settlements – 64 pre-generated settlements for any setting.
Kingdoms & Warfare – A sourcebook from MCDM on establishing your own fiefdom (whether it’s a noble court, monastery, thieves’ guild, etc.) and raising your own personal army.
Campaign Builder: Cities & Towns – Kobold Press’ guide to adding pre-generated locations and NPCs.
Deep Magic I and II – A pair of magical tomes from Kobold Press with new subclasses, spells, schools of magic, and other fun goodies.
Tal’Dorei Campaign Setting Reborn – An excellent resource for Critical Role fans, this would pair perfectly with the official Netherdeep and Wildemount books for 5e.
Crooked Moon – A folk horror setting and sourcebook that I mentioned on my other thread.
The Star-Shaman’s Song of Planegea – Also mentioned on my other thread, this is a unique prehistoric campaign setting with dinosaurs and megafauna.
Ruins of Symbaroum – Also mentioned on my other thread. An excellent Symbaroum campaign setting by Free League.
Iron Kingdoms: Requiem – An original steampunk campaign setting from Steamforged Games.
Weird Wastelands – A sourcebook with ideas for how to turn your fantasy world into a post-apocalyptic wasteland.
Planet Apocalypse – Another sourcebook with ideas for how to turn your fantasy world into a post-apocalyptic wasteland.
Obojima: Tales from the Tall Grass – A Studio Ghibli-inspired campaign setting mentioned on my other thread. I’m definitely not the target demographic for this, but if you are then you should definitely check it out.
Ryoko’s Guide to the Yokai Realms – A beautiful sourcebook in an east Asian-inspired setting. Lots of enormous kajiu to slay.
Cthulhu Mythos – If you’d rather get your Lovecraftian fix from 5e instead of the Call of Cthulhu TTRPG, this one is for you!
The Compendium of Forgotten Secrets – A sourcebook with otherworldly patrons and abilities for warlocks.
The Ultimate Adventurer’s Handbook – Pretty much what it says on the tin. More races, classes, sub-classes, feats, spells, and so forth.
Book of Spirits – A sourcebook with 50+ spirit creatures, spiritual boons and banes, new classes and races, even three pre-scripted adventures. Beautiful artwork.
Kibbles’ Compendium of Legends and Legacies – New classes, subclasses, feats, and spells.
Kibbles’ Compendium of Craft and Creation – New classes, subclasses, feats, spells, monsters, and a crafting system.
Kibbles’ Compendium of Airships and Artifacts – Another book from Kibbles with new classes, subclasses, artifacts, and a whole airship system.
Exploring Eberron – An Eberron sourcebook that would mesh really well with the official 5e Eberron books.
Frontiers of Eberron: Quickstone – A deep dive into Eberron with new species, subclasses, feats, items, etc.
Dark Matter – A sci-fi sourcebook and setting from Mage Hand Press.
The Gunslinger Class – A new class with new subclasses, firearms, and feats.
The Dragon Knight Class – A new class focused on pacts with powerful dragons.
Ancestral Weapons – Rules for creating and upgrading ancient and legendary weapons.
Humblewood – A woodland setting from Hitpoint Press featuring anthropomorphic animal heroes.
Steinhardt’s Guide to the Eldritch Hunt – One of the most grimdark folk horror sourcebooks out there, this one combines Lovecraftian themes with Soulsborne elements.
Battlezoo Eldamon – This setting is essentially a fantasy version of Pokémon with loads of unique monsters to collect.
Dr. Dhrolin’s Dictionary of Dinosaurs – A bestiary of dinosaurs to include in your campaigns. You can finally achieve your dream of becoming a Dino-Rider!
Compendium of Beasts – A homebrew bestiary by the legendary LaserLlama.
Compendium of Beasts: Dinosaurs – A sequel to LaserLlama’s Compendium of Beasts, this one is all about dinosaurs!

Edit II: More Recommendations From Kingmaker:
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Strongholds & Followers – A guide to creating castles and fortresses in your games, as well as some NPC options to fill them with, plus a stand-alone adventure. A companion volume to Kingdoms & Warfare above.
Broken Weave – A guide from Cubicle 7 on how to make your medieval fantasy worlds grimdark and post-apocalyptic. Yes, one more post-apocalyptic wasteland guidebook.
Uncharted Journeys – Cubicle 7’s 5e rules for traveling and launching expeditions, plus nearly 2,000 pre-generated encounters.
A Life Well Lived – Cubicle 7’s companion volume for Uncharted Journeys with material for developing backstories, character activities before and after adventures, ideas for character actions during long rests, careers for players to develop outside of dungeon-crawling, and how to establish a home base.
Hammer & Anvil – Cubicle 7’s rules supplement for crafting weapons, armor, and other gear in 5e.
Mortar & Pestle – Cubicle 7’s alchemy rules supplement for 5e.
The Complete Armorer’s Handbook – Rules for crafting, repairing, and upgrading equipment. Plus rules for wear and tear on your gear, and even rune-smithing magic.
Heliana’s Guide to Forgotten Monsters – A sequel to Heliana’s Guide to Monster Hunting, now on Kickstarter.
Ryoko’s Guide to Forbidden Myths – A sequel to Ryoko’s Guide to the Yokai Realms, now on Kickstarter.
Altheya: The Dragon Empire – A new high fantasy setting that’s just wrapped up its Kickstarter. Fate mechanisms, relics, plot hooks, and of course, dragons. The art is truly beautiful too. This one looks really good.
City of Arches – A fascinating sourcebook by Sly Flourish about a setting-agnostic metropolis built at the conflux of many gateways to other planes.
Fantastic Lairs – A sourcebook of pre-generated dungeons and boss fights by Sly Flourish.
Fantastic Locations – A sourcebook of pre-generated locations by Sly Flourish.
Prothero’s Guide to Spellweavery – A further expansion of the rules for magic and spells.
Where Evil Lives – A companion volume to Flee Mortals!, this book is all about boss battles.
Exordium: Origins of Mythos – A rules codex for epic tier characters with multiple mythic paths to choose. A truly beautiful-looking volume.
The Seeker’s Guide to Twisted Taverns – From Ghostfire, this sourcebook has 17 whimsical taverns with quest boards for each of them offering unique adventures.
Armaments of Legacy – This supplement adds legacy weapons, epic items that evolve with your characters over time.
Artifacts of the Guild – A collection of the top artifacts produced by authors at the DMs’ Guild.
Tatters of Myth Drannor – A new sourcebook for the legendary elven city of Myth Drannor.
The Whitefaire Adventurer’s Guide – A sourcebook for the savage northlands.
Journeys Beyond the Radiant Citadel – A sourcebook companion volume to Journey to the Radiant Citadel.