I started this thread so that we could share books we, personally, have found useful to the writing process. Please add yours!
I say "encyclopedia type" because they contain a lot of information on a particular topic, arranged alphabetically by subject. Sometimes you need a book that's more detailed in certain areas and contains absolutely nothing about most other areas, yet is more... able to be dipped into than an historical treatise on your subject. This is one I like.
The Element Encyclopedia of Witchcraft by Judika Illes. My edition now out of print, but the reason I like it is that it is an idea-spawner as well as a place to look things up. Going to a book like this, you always find far more than you wanted to know.