There's "Writing and Selling Your Mystery" by Hallie Ephron, but as I recall (and it's been quite a while since I read it), it didn't really address cozies specifically, and much of the focus was on issues that are more important for, say, procedurals, where you can't just sort of say "the police did stuff while the protagonist went her own way," instead of getting detailed about the police actions.
I was never able to plot a coherent romance, but plotting mysteries comes (more or less) naturally to me. They seem to follow most standard plotting methods, although you may need to experiment with assorted ones to see which ones (or which parts of them) work for you.
I've used things like the Snowflake method, or a variation I call the Pizza Method, which sort of combines the basic Snowflake method (start with really brief elements of plot and characterization and premise, and then keep expanding each one) with some other ideas I've run across, including the Rule Of Seven, which I saw in guest post at John Scalzi's blog once, which forces me to have seven bits and pieces before I start writing -- a killer title, a premise, a subplot issue, a non-mystery goal for the sleuth (like, in Donna Andrews's books, the sleuth will be organizing a yard sale or a family reunion or something, while also solving the murder), a motif (an inside joke or some such thing) that runs through the story, and a compelling aspect of the antagonist.
I also found some of the stuff at Larry Brooks's storyfix.com to be useful. (Go to the oldest blog post and work your way forward.) I might quibble about where some of the plot points hit in a cozy occasionally, but mine are usually pretty close to where he says they should hit, although I might define them a little differently.
I think one of the best things to do is to define what a satisfying plot looks like FOR YOU, in terms that make sense TO YOU. It's a little different for everyone.
Take a look at your favorite cozy authors and see what percentage of the story you're at when 1) the body is found, 2) when there's an escalation, and 3) when the crisis occurs. Establish the turning points for the story, then look for where the various suspects were introduced.
For me, the dead body generally shows up at the end of the third chapter (which is fairly standard, although a lot of Donna Andrews's show up later, and some have it on the first page). My first act generally doesn't end there, but the turning point is when the protagonist decides to investigate. I make sure all my suspects are introduced in the second act, usually the first half of it. I like Larry Brooks's suggestion to have a point ... I forget where he places it, although it's in the second act, toward the end, I think; I have it in my outline template ... where the antagonist demonstrates his/her power (i.e., things get more difficult for the sleuth, and there might be an attempt on her life).
Then figure out what YOU like as a reader. Do you want the murder on page one or do you want a little bit about the community of characters first? How much escalation will there be? Figure those things out for what YOU consider a well-plotted novel (within the limitations of what you've observed other successful authors do), and then go with it! (Jack Bickham makes a similar suggestion in his book on Writing and Selling Your Novel -- figure out a novel structure that makes sense to you and that works for you, rather than blindly following a structure someone else imposes. It does require that you understand and be able to identify story elements in the stories you study first, though.)
Molly! Glad you're here. 
