Speaking as an occasional author, by the time I've finished the however-many drafts and several rounds of edits, I'm completely blind to the manuscript's mechanical flaws, and need someone else to point out to me the Spoonerisms, unfortunately euphemistic word choices, or even just that I've dedicated my book to "My parents, Ayn Rand and God."

Also, I say again: copyeditors and proofreaders are cheap. Doing that work myself makes about as much sense as doing anything else I could outsource cheaply.

What kevinr says. At some point, you start reading what you expect to read rather than what is actually on the page--including typos with red lines under them. You basically cannot publish something significant without having another set of eyes on it at a minimum. A decent copy editor will also help fix up some things (in my case excessive wordiness). Tools like spellcheck help but only to a degree. Also as kevinr says, basic copyediting is pretty cheap. I paid about $400 for my last book.