I just finished reading Stephen King's On Writing - which is a part CV part How To guide. He specialises in Science Fiction, but most of his advice to wanna-be authors is about style and form which can be applied to crime fiction. He also offers tips on how to keep focused on the job and avoid writer's block and stale characters. It helped me a lot.
So, I was wondering - what was the best How to Write book you've ever read? Why was it helpful? Have you even read one before?