The Tough Guide to Fantasyland - Diana Wynne Jones This is hilarious, an absolute must for every fantasy writer. The book is a mock A to Z guide of the tropes of fantasy. Now and then, I just open it randomly for a dose of laughter, read a few entries starting with different letters, giggle, and close it again, till next time.
As I writer, I can say that if you write fantasy, you can't avoid at least some of the clich├ęs described in all their ridiculous details in this book. It's up to you to use them in an original way, if at all possible. Of all the fantasy writers I've read, only Terry Pratchett succeeded at this task.
I'd love to quote from this book, but it's practically impossible to choose the best quote. The entire book is a book of quotes.
Marvelous and entertaining. Read it!