I'm going to second some of the suggestions you've already received, and then add some of my own.
One of the best starting points for me was The Elements of Mystery Fiction by William G. Tapply. In addition to quite a bit of advice for writing mysteries, it also gives you a lot of general writing instruction. It is a good place to start.
I also loved Writing and Selling Your Mystery Novel by Hallie Ephron. It is just packed to the brim with helps to get you started and make you think about things that as a reader, you might never think about. It is more of a step-by-step guide to the whole process. Very helpful--but you might want to balance this book with others to avoid the cookie-cutter mystery.
I also picked up Don't Murder Your Mystery by Chris Roerdon. I haven't read this completely yet. I plan on doing that after my rough draft, since it seemed like many of the suggestions could best be applied to the editing process.
I've also read portions of Sue Grafton's Writing Mysteries. That's really a collection of stand-alone articles by various writers. It's nice because you can pick an area you might be having trouble with (or just want to know more about) and read only that chapter.
But I really have to echo what Clair said, and tell you to read in the genre--and study what you read. The mystery-writing books will make you even more aware of what to study, since you will then read books and examine everything from POV to how viable the red herrings were, to how well hidden the clues.
I do have to warn you though, it takes half the fun out of reading mysteries after a while. After you know how it is done, you start figuring out whodunnit and why a lot earlier than when you just read casually.
Good luck with your cozy. That's what I'm working on too. I love the genre.