While that's not really what they do, perhaps Tor Books[1] employees might have a perspective on this? Personally I think it would be really hard to do a good work of fiction without some help -- it is of course possible to hire an editor by yourself -- but I think the kind of support a "real" publisher is able to give you would be really valuable in this case.

[1] http://www.tor.com/ (For those unfamiliar, Tor was very early with DRM-free e-books as well as offering the first books of series for free download (expecting readers to purchase following books)).