Xanathar's Guide To Everything
Wizards of the Coast
XGE is a non-core sourcebook for 5th Edition D&D that was just released in November 2017. Despite being totally optional, I think it’s fantastic–I’d recommend getting it before the Dungeon Master’s Guide (although the magic item list is important, there’s a pretty decent list in the freely available System Reference Document). It has a bunch of optional class archetypes and spells. But the most compelling part for me is the “This Is Your Life” section, which has a lot of aids for fleshing out your character. In my experience, both as a player and as a DM, players tend to spend most of their time building their character’s class and abilities more so than his or her personality and backstory. I think the game is a lot more fun when the latter come into play, and I think this section can help with that. The other most awesome part is the section on “Common Magic Items,” which describes a bunch of things with very weak powers that are just full of flavor, and I want all of them in my campaigns.
My only real criticism is that the “Character Names” appendix, particularly the 10+ pages of human names, is obviously just there as filler–it’s easy enough to find a million websites with lists of human names!