This wonderful book is easily the most helpful thing I've ever read for the horror author. In essence, it is a collection of essays from various writers about writing speculative fiction and dark fantasy, with chapters by the likes of Ray Bradbury, Robert Bloch and Charles Grant. Much of the lessons do apply to Sci-Fi/Fantasy as well, although if that's your concentration I would recommend Orson Scott Card's "How to Write Science Fiction" before this.
The book is a brisk read right up until the chapter on the psychology of horror fiction which, frankly, reads like it was written by a psychologist. It was boring, tedious and felt very out of place among so many good essays.
The only other problem is the chapter at the end that supposedly helps you get published. While I'm sure it was quite helpful at the time it was written (in the mid-1980s), many of the publications it lists are now defunct and many of the practices have changed. It's not a bad section, it is simply past its time.
Regardless, for those who want to learn how to write horror from those who have done it, this book is well worth reading.