The Publishers Research Group, pubresearchgroup.org, has published its first survey of POD publishing. You have to register and you may be surveyed, but this is a very fine, unbiased survey that will be very useful for you as an author.
My POD publisher, booklocker.com, was not listed, but included in “other” because I completed the survey. Angela Hoy, BookLocker's owner, puts out a weekly newsletter, WritersWeekly.com that is very fine and informative with the largest circulation on the Internet. I have been working with her and her husband, Richard since 1999. They are very inexpensive and honest. As a result, they don't publish any poorly written work (most of the top ten do, without editing if you pay them) and they don't publish poetry.