Look, folks. While there are some people who can do it all, the reality is that publishers can do things that the writer hasn’t the skills, time, or desire to do. Middlemen exist and succeed because there are so many moving parts to handle in just about every business, not just the book business.
What Penelope gives is a partial list of what the publisher should be doing for you, plus a couple of, well, publisher specific pitfalls. In addition a publisher should be thinking about “printers” – whether owning them like the Big Boys, or contracting with independent presses, and whether it is audio or paperback or hardback or electronic or, well, you get the idea. The publisher needs to have thought of the alternative when BigPrint decides to double the print charge, or when OtherPrinter has a fire that destroys their press. Thought about it and made enough prep that the delay in readers getting books is not more than a couple of weeks. A month? you just killed your author’s audience and shattered your professional reputation.
You should have artists and voice actors and typesetters (or their epub equivalents) stuffed into your contact list along with database houses and marketers and printers and… Sure, you the publisher might indeed be able to do it all. But if you’re any good you’ll have too many books to do it right for each. You should have your IP attorney and your contract attorney, and it probably wouldn’t hurt to have an attorney’s name for every state and nation in which your books are sold, just in case.
You are offering yourself as the person who gets your authors’ books sold so the writers can concentrate on writing. Some writers will want more involvement, others none, but that is still your job regardless; it is what you are supposed to be bringing to the deal.
If your author’s books aren’t selling, you failed. Fix it.