Leomund's Legendary Lexicon of Lost Languages, a book on an often overlooked aspect of the game: the language, alphabet, famous written documents and known expression of every races.
Evard's Enigmas (a sourcebook of riddles and puzzles to baffle your players, with tables for rolling up your own random ones)
Farming For Fighters (a self-help guide to downtime activities for when your hero retires)
Mordenkainen's Mystix Manual (the complete guide to psionics that everyone's been waiting for)
Otiluke's Options (hundreds of optional rules to help you homebrew your own)