1e: Unearthed Arcana. I can hear howls and screams now, but it was a book that injected many new ideas into the game, even if all of them didn't land.
2e: Complete Book of Humanoids. Some of the implementation was wonky, but this book really got creative with the kinds of characters one could play, doing more to kill the same boring core races than any other (outside of maybe the Planeswalker's or Complete Spacefarer's Handbooks).
3e: Unearthed Arcana (again). There were several great books in the 3.5 era for changing the game's paradigm (Tome of Magic, Tome of Battle, Savage Species, Magic of Incarnum, etc. etc.) but this one had more cool ideas than any one game could ever use, and there was a decent amount of explanation as to how this could effect your game. Some duds, like Bloodlines, but many of the alternate classes and features (Druidic Avenger, Cloistered Cleric, Feat Rogue, Rage variants, Totem Barbarians, etc. etc.) were a breath of fresh air.
4e: Player's Handbook 3 (man I miss the days when we had multiple PHB's and DMG's!). There's a lot of books that added new content to the game, some good, some meh, but this one rises to the top for probably the best multiclassing D&D has ever had, hybrid multiclassing. You feel like a this class/that class from level 1, much like AD&D, but it's balanced in that you don't get everything, and you don't have a ton of limitations holding you down like slowed advancement, fractional hit dice, level adjustments, or any of that. Some of my most fun characters were hybrids, like my Bard/Wizard or Blackguard/Assassin.
5e: it's hard to draw the line between innovation and power creep. Sword Coast Adventurer's Guide was too little, Tasha's Cauldron of Everything many feel is too much. So I'm going to go with Xanathar's Guide to Everything as a book that examined the game, discussed some points about it, and tried to fill some gaps, without making everything before it "old and busted" compared to the "new hotness".
