To add to the comment form nickkokonas, I'd also add that there is a distinction between editing/tech review and copy-editing. They can be done by the same person, especially if you've had at least some serious read-throughs by people you trust who are familiar with the field.

But you basically need a content review and a review for flow/grammar/spelling/etc.

I've self-published but my objective has never been to make money. It's about supporting other activities that I get paid for. (Which is the situation with a lot of the lighterweight O'Reilly and "Dummies Guides" types of things these days.)