Go and read this blog on fantasy faction:
This is a very good, very informative blog. I self-published my first novel last year, but had to take it off smashwords.com as I’d very quickly got a three book deal for my trilogy, of which The Cracked Amulet is the first.
Katie Isbester, chief editor of Claret Press, London, loved my book enough to give me very constructive and incisive criticism and advice to make my book vastly superior. She has an eye for detail in both the narrative and in character development that helped my story become both more intense and readable.
Cracked Amulet is now going through the professional design and layout process and will soon be published.
As Marc says, a professional editor reading through and advising on a book is priceless and should not be ignored no matter what route is taken towards publication. I learned a fair amount from my editor and am very, very pleased she took me and my scribblings on.