One of my favourite reference books is The Book Of Imaginary Beings, by Jorge Luis Borges.
If I have a problem with the fantasy genre at the minute, it is that, on occasion, it does not embrace… erm… fantasy. It can be quite conservative in the imagination department, and I wonder where the fantasy has gone. Well, this book is chock-full of great creatures that can be referenced. It is a bestiary of mythical creatures, including ones I’ve used, banshees, garudas and sirens, covering locations all over the world, and their origins. It’s not even that big a book, so it really is something worth picking up for any connoisseur of weird stuff, especially for writers, and then there really is no excuse for having standard fare!