Over at the Religion in American History blog, Randall Stephens offers some sound advice on academic publishing in his post, "Turning It into a Book."
There are several good books for prospective authors, but the one that I followed was Beth Luey's Handbook for Academic Authors. I simply bought the book and followed the step-by-step instructions for putting together proposals for academic publishers. This was the only source that I used, as it offers comprehensive advice on how to break into the academic publishing world. I sent out close to a dozen letters of interest and proposals, received four yeses, a few maybes, and the rest were nos.