>there is no mention of getting someone to proofread or even copy edit the book. This would also seem really important for a programming book - e.g. to check that the examples work away from the author's dev environment.
You're actually talking two different things.
Someone needs to proofread/copyedit the book. Full stop. And, unless you have a partner or particularly close friend who will/can do a careful edit for you for a case of beer and a pizza, you're going to have to pay someone.
For a programming book (or other types of technical books), you probably need a technical reviewer. If it's just a sanity check for technical accuracy, colleagues etc. can probably do that. But to work through all the code in a book, again, someone will probably have to get paid.