Check out Jimmy Akin's podcast, (I believe it was the second question, about 1/2 way through the podcast) in which he discusses Canon Law about when to seek the Imprimatur. He's addressing a question about blogs, but he discusses printed material.
The Catholic Writers' Guild has a Seal of Approval they will give books in line with Catholic teachings. If you're a Catholic writer, go check out the Guild website to learn how to apply for the Seal and about how to join the Guild. They're a wonderful organization.