What's the most useful non-gaming source of information (book, website, etc.) you've ever found? And what makes it so darn useful?
My funnest and most useful non-game book has been the Encyclopedic Psychic Dictionary by Bletzer.
In my gaming, terms and approaches to the tangibles are all around us and we can talk to fellow gamers easily about Strength, Endurance, etc. Talking about intangibles is a lot harder. Bletzer's dictionary provides scope without prejudice.