Hi, Vicki!
Thanks for dropping by. I have a long list of books I recommend for mystery/ suspense authors...

Anything by James Frey; he wrote "The Key", "How to Write a Damn Good Novel", and "How to Write a Damn Good Mystery";
"Writing the Thriller" by T. MacDonald Skillman; this explores the various subgenres - technothrillers, legal thrillers, medical thrillers, cozies, etc;
Something in addition to "Building Believable Characters" that I can't live without is the Writer's Digest Flip Dictionary; when you just can't remember that specific word, or you need to know the parts of something, like a gun...
Generic books I think are invaluable for anyone trying to get published are "The Publishing Game" books - How To Find an Agent, How to Publish a Book, and How to Have a Best Seller. At the end of each subtitle is the phrase "is less than 30 days" -- forget that, but they still have great advice.
And of course, I always keep a baby name book close by;
And believe it or not, I use Linda Goodman's Sun Signs, Moon Signs, or Love Signs. I don't really believe in astrology but they are terrific at building personalities in your characters, especially when you have two people in conflict, like a married couple or boss/subordinate, siblings, etc. Lots of great ideas in those books!
I hope this helps... I have literally about 100 books just used for reference here in my office, but those are the main ones I'd recommend.