One of the latest major supplement for the 5th Edition of Dungeons and Dragons, Xanathar's Guide to Everything, is a grab bag of all sorts of odds and ends. While not a necessity in the strictest sense, the rulebook provides a whole lot of flavor to an already impressive line.
Though the supplement's name does it no favors, it's mostly a reprinted collection of an online article series with some added depth. Having it all in one physical place, however, is helpful. Even so, the game of D&D technically only requires three books: the Player's Handbook, the Dungeon Master's Guide, and the Monster Manual. But if there's a fourth book every tabletop group should pick up, it's probably Xanathar's.
The question most groups will likely be asking themselves is whether the $49.95 MSRP is worth the sticker price. Well, good news! The book is under $30 at many retailers, and it nearly doubles the amount of options for individual players regardless of class while providing guidelines for a number of tricky situations as well as expansive flavor material like origins and life events.
The reality is that Xanathar's, used correctly, is basically a nonevent for any tabletop game. The material it presents should seamlessly integrate into any campaign, even if it's introduced in the middle of things. While that might turn some folks off, it's honestly the best possible outcome for any supplement like this. It's interesting without being overpowered, and everyone gets a few more official rules to play around with on the regular.
It might not really be a guide to absolutely everything, but there's certainly enough here for everyone.