But be mindful of the fact that a manuscript is not a book. Every writer needs to have their manuscript edited before it hits the presses, and the process often takes longer than the actual writing of the manuscript. If you know a skilled editor who will do it for free, great. Otherwise, hire one or sign with a publisher. Just don't skip this step, eBook stores are filling up with texts that are barely readable.
... and some more than others.
Please be realistic about your ability to write clear and concise English. I was recently acting as a technical editor for a book, and at times found myself saying things like "it's not clear what the author is trying to say here; he might mean X, he might mean Y, or he might even mean Z -- but all three are wrong, because the correct statement is actually A".
If you don't already write for a living (either in the literal sense, or as a researcher or in some other profession where you regularly produce 10+ page documents of English text), start by writing a blog.
A good editor is someone who knows good writing and will tell you if you're producing crap. They should also be able to tell you if you're writing to the current zeitgeist.
There's value to having good editorial support but, IMHO, it's not worth signing over all your rights and earning potential.
You can outsource all phases of editorial and production, the big houses already do.
I realized this the first time with reading the "uncut" version of Steven King's The Stand. The original published version was one of my favorites, so I figured that the extra-long version of the writer's original vision must be even better. Wrong. There were very good reasons why the book was edited the way it was.
The moral is that even the best-selling, most celebrated of today's authors need the help of an editor to help shape things properly. Don't think you're any better.
He goes into process of writing a book with razor sharp detail.
You can self-edit, but you need some distance between the time of writing and editing. Also, editing technical work is different to, say, a novel. You also need to know your written language very, very well. And you need to know fundamental typography, which differs from country to country.
So, yup, there's a lot more to publishing than scribbling out the words and throwing out there.
The trick is learn to find two sets of readers. One who just love reading and can articulate what they like and what is lacking in a story (they really wanted this plot thread to finish but it kind of died, that sort of thing) then a group to help catch grammar and punctuations mistakes. After that if you think it's worth the cost (aka you think it has a shot of doing well) paying the ~1k+ for a professional editing service to have one last go over it is a good idea since they do that sort of thing for a living.
Now if I could just find my two sets of readers once I finish the second draft of my novel...
I have never attempted to write anything book length, but I have published some short technical articles, and it is always clear which ones benefitted from a good editor and which ones did not.