Every printer that exists has a prepress department. Every printer that exists has a proofing/editing department (which all too often falls on the CSRs for that printer). So how is a publisher different from a printer?
I have worked with both. At my previous job I worked for a magazine publisher. My job was to train staff across the country on book formatting so when that regional office sold ad space they kept in mind the layout necessary to work for their regional magazine. We are talking spreads here and signatures. I had a lot of fun with that job, but realized quickly that I needed to know more about the printing side of the business. I can tell you that nothing drives a printer crazier than a graphic artist that does not understand printing. They will send over beautiful designs for marketing collateral just to have us turn around and say - this is set for RGB, you didn't add a bleed, your dimensions are off, your fonts are not embedded, and etc, etc, etc.
So I left the magazine publisher and have now worked with a printer for almost 5 years. I love my job. Most of my clients that need things printed - training manuals, marketing pieces, signage, business cards, and so on. For these clients we are the publisher and the printer. My job is the help the client develop his or her ideas and then work with prepress to get a design going. In many ways, most printers correct the issues that publishers/graphic artists/ad agencies took forgranted.
Book publishers are very different from magazine publishers. Their job is to scout out talent, pretty up the material (manuscript), and format it for the printer. You should know, though, that the majority of the time the printer still has a lot to do to work out the kinks for your book before it goes to press. Font issues are always a problem, which is no doubt why agents typically prefer Times or Courier - they are fail-safe fonts to them - but once the job goes to the printer there is no such thing as a fail-safe font. There are a zillion versions of Times and if the printer does not have that font then the font defaults - typically to Courier or Arial. The flow of the entire piece alters and the printer is left looking at a completely different layout than what the editor/publisher intended.
The next issue would be the jacket cover. Okay, I imagine that most printers cringe when they receive these files. Pull a book off your shelf and examine it. Every single object, line of text, photo, etc is a separate item in the design document used to create the cover. Type must be made in Indesign or similar program. Photos set in Photoshop. Vector objects in Illustrator. Then everything must be placed into the main layout program - we primarily use Indesign. If any one of the aforementioned items is not linked properly then the document with error out when saved. The non-linked item then looks like a low res image - blocky/fuzzy and just a plain out mess.
Finally the book has to be perfect bound and here we have a new dilema. The binding of the book has to be shaved inorder to create a clean surface for the glue. If the original designer did not layout the book properly then the text/words are too close to the spine - ever read a book like this? I have. Isn't it frustrating when you have to flatten the book out more to make it easier to read? This is a design flaw, not a printing flaw. Then the cover gets glued onto the text and again, we have potential problems. The remaining 3 exposed sides have to be trimmed off so you have A. the cover bleeds off the page properly and B. the edges are smooth and to size. If the cover design was not dimensionally correct text/images/etc are off center. The entire book would have to be reran.
So next time you take a book in your hand take a hard look at all the elements necessary to make that perfect piece of art in your hands. :)