Dungeons and Dragons RPG: Xanathars Guide to Everything
Looking to make an extremely long dungeon/adventure for your players to run around aimlessly in? Then this is the book for you!
There are a plethora of tables to create random encounters in this book that can help enhance your game, or just give your players something to do while you think of the next part of their journey.
Xanathar's guide also details over twenty-five new subleases, which makes it a great book for players as well as Dungeon Masters.