[ghaff]

>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.

[TheOtherHobbes]

This is a much more useful comment than the OP.

Books need review - not just a few comments on the general concept, but line by line proofreading of the English, and fact checking of every single technical statement.

This is fundamental to the process. It's not an aspiration, it's part of the writing process - because often you'll be writing content while other content is being reviewed, and feedback from both can influence the rest.