If you were playing Dungeons & Dragons in the early 2000s, you probably remember the sheer volume of books - both official and unofficial - that were available for the d20 System. If you wanted something in your campaign, there was a good chance that you could find a complete supplement with hundreds of pages of lore, inspiration, and game mechanics to use.
The approach to the modern incarnation of D&D is a bit more reserved and calculated, so the days of filling an entire bookcase with official material are likely over. That's a shame, because these 3E books deserve a 5E makeover.
Monster Manual 2
And 3, And 4...
The Monster Manual is an iconic part of D&D, so much so that it's one of the three core rulebooks. Stacked front to back with stats for foes to challenge adventurers of all levels, it's a DM's best friend and essential reading for players who want to know what they could be up against.
Every version of the game since AD&D has introduced additional Monster Manuals, starting with the Monster Manual 2 all the way back in 1983. 5E, though, has been going for over a decade, still with just the one. Sure, there's Volo's Guide To Monsters and a few other books that have additional beasties to battle, but the modern game needs full-on tomes of enemies as its popularity continues to grow, to keep encoutners from getting stale if nothing else.
Expanded Psionics Handbook
Lae'zel Approves
Sorcerer this, Warlock that, I say bring back psionics! As they existed in 3.5 they were kind of clunky, but with a rules overhaul they could be brought into 5E to provide exciting new options for players who are tired of just casting Eldritch Blast every round.
With the cultural prevalence of the gith and illithids thanks to Baldur's Gate 3, there's no better time to make psionics a part of D&D again. Better yet, longtime players would appreciate the return of a classic sourcebook.
Lords Of Madness
Make Wild Magic REALLY Exciting
Speaking of illithids, one of 3.5's best supplements was dedicated entirely to aberrations - extradimensional invaders and the terrifying byproducts of Wild Magic gone wrong. Aboleths, beholders, and more made for reality-shattering threats that brought tension and danger to the table in equal measure.
D&D 5E has plenty of undead, goblins, and demons to fight. Adding in a whole book's worth of aberrations - plus reasons to make sorcerers think twice when they try something risky - would spice up adventures in lots of exciting and unexpected ways.
Sandstorm, Cityscape, Frostburn
Biomes & Bugbears
These 3E supplements went in-depth on specific environments; deserts, dense cities, and arctic biomes, respectively. They were fantastic for adding flavor to specific campaign locations, though obviously their laserlike focus made them hard to use elsewhere.
You haven't lived until you've sprung a mummified kraken on your players in the middle of a desert.
These books offered a wealth of inspiration for DMs and players alike, whether they were looking for backstory, new encounters, or just immersive concepts and illustrations. Even if you aren't planning a game in one of these settings, reading a book like this might push you in that direction.
Book Of Exalted Deeds / Book Of Vile Darkness
Fantasy Has Always Been About Good And Evil
There's a lot to be said in favor of D&D's shift in approach toward Alignment in recent years. It's particularly notable in its effort to move away from the idea that members of a species are inherently good or evil - something that fantasy as a genre has always struggled with. Even with the game's re-examination of older tropes, though, there's still room for the interplay between good and evil, especially when a character or kingdom falls so heavily onto one side or another that their Alignment isn't up for debate.
The Books of Exalted Deeds and Vile Darkness explored the two sides of the coin, letting players and DMs make the most heroic heroes and villainous villains. How we use the dichotomy in media, including roleplaying games, might evolve, but exploring what they mean must always be an ongoing process.
Ghostwalk
The Coolest Option Nobody Used
Possibly the only "Campaign Option" sourcebook (if there are others that I'm forgetting, let me know in the comments), Ghostwalk was built for high-concept storylines. It was designed for your group to make a campaign where the player characters started off dead, and now wandered the world as ghosts.
It was an incredibly cool idea, and the spectral hub-city of Manifest could be plopped into any existing setting with relative ease. Of course, it was also very niche, but given the success of Curse of Strahd, there's certainly room for more undead angst in today's D&D.
Heroes Of Battle
Grond! Grond! Grond!
3.5 existed at the same times as the Lord of the Rings films and the many, many movies of the decade that tried to capture the magic (and box office revenue) of the trilogy's big, epic battles. Naturally, D&D groups also wanted to have their own Helm's Deeps and Pelennor Fields, and Heroes Of Battle provided all the mechanics to do just that.
Not only did Heroes Of Battle present rules for mass battles, letting PCs take command of units or fight themselves against overwhelming odds, it also hearkened back to D&D's origins as what was essentailly a mod for the miniature wargame Chainmail. There's absolutely a place for heroic sieges and decisive melees in 5E.
Draconomicon
Just Having It On The Table Was Enough To Make Players Sweat
Let's be honest; if you're lucky enough to play a full campaign of D&D, going all the way from 1 to 20, but in all that time you never face a dragon, you're going to feel a bit cheated. Dragons are the iconic boss monster of D&D; they're right there in the name, after all. Draconomicon: The Book Of Dragons provided everything you could have possibly needed to know about the great winged beasts.
This gorgeous book was brimming with lore and stats for every type of dragon imaginable, allowing for encounters at all levels. It deserves to be a perennial title in the D&D lineup, but so far, it's never seen an update for a later edition.