Alphabet Soup is a comprehensive guide to words commonly (and uncommonly) found in games or used by gamers. It provides quick, easy definitions in lexicon form: gaming terms, definitions, historical context, mythological terms. It even gives gamemasters advice on how they can use them in their games. It works like an encyclopedia of random bits, only these random bits are things I think are interesting for gamemasters.
In this particular edition, I've provided a 1d100 chart of magic rings (and their side effects) that will flesh out any fantasy game.