As far as craft books go, How to Write Tales of Horror, Fantasy, & Science Fiction edited by J.N. Williamson is a bit outside the normal variety. This book actually houses a series of essays on writing "Speculative Fiction" as a whole genre. Whether we as Fantasy or Science Fiction writers think about it, the fact is we need what the Horror writers have: the ability to make readers worry. The bulk of these essays will tell you just how to do that, using the fears you already have locked inside your own mind. If that sounds scary, then you're off to a good start. If you feel the fear in the scene, while you're writing it, then you have good odds of a reader (who is NOT in control of the story) feeling that same level of fear.
"Redream the Fictive Dream" as we learned in Gardner's The Art of Fiction. This is a key harped on again and again by the masterful writers in these essays. There's also a really handy list of publications for SF&Fantasy magazines (some of which are out of print now, but that's what libraries are for). In short this book is chock full of useful bits of information beyond just craft. These writers knew the business in 1987, and much of their advice still holds. The biggest piece is: write the absolutely best story you can.
If you're looking for a craft book that you can nibble in convenient pieces, then this is a solid choice. The essays by Koontz and Bradbury are always a joy to read, and some authors you might not even know have pieces that easily compete with these two masters.
Not all of us can have a mentor in writing, but thanks to books like How to Write Tales of Horror, Fantasy, & Science Fiction we can have something like a mentor (or a dozen mentors).
As always, thanks for reading. Now get back to writing.