The Craft of Writing Science Fiction That Sells
Ben Bova
Writer's Digest Books
I found this book to be most instructive.  It offers practical advice and a checklist of things to look for when writing a novel.  In fact, it is valuable advice for any type of novel, not just a Science Fiction novel.  But if you are interested in writing Science Fiction, there are valuable details that you won't find in other books.  If you want to write either a novel or fictional short story, this is a must-read.