This isn't as good as Barron's volume on SF, Anatomy of Wonder. I haven't seen the 1999 edition that combines fantasy and horror, but it would need a new edition too: the genre has really grown in the last ten years.
Annotated guide to genre, indispensable to libraries, with key entries rated, author studies, introductory chapters, and author collections archived where. Wealth of information and for readers wanting to broaden tastes.