Kevin DeYoung just wrote a very sensible piece on the “state of Christian publishing.” Worth checking out, especially point 5:
I’ve seen many books in the past few years that I would put in the category “Really good stuff, but I’m not sure it was book worthy.” These are books that might have been excellent sermons or terrific blog posts or could have been a wonderful long article, but a stand alone book they feel underwhelming.
This aligns well with the things Seth Godin says in his article on tracts and manifestos. Sometimes we just stretch content to thin for its own good (you know who you are, Peter Jackson).