This is more a question for a publisher, I suppose, but how much "fixing" of a manuscript can be done when it's in galley form? I'm thinking in terms of a non-fiction book in which circumstances are likely to be different by the time the proof is done, and it might be necessary to add some material, not more than one page. Plus, little diddly things, rewording a phrase, adding a sentence here or there -- is that reasonable? Or should one only count on making changes necessary to, say, dodge a libel suit?
Don't plan to change anything once it hits galley stage. Fixing typos is one thing but you start mucking around with page order and they're going to send you an invoice.
Your publishing contract will cover this. Standard terminology is usually they'll fix errors but not something you change your mind about.
However...there are several stages before you get to galleys that allow for all sorts of changes. There's time between "yes we want to publish this" and "here's your book" to fix all sorts of things but earlier is MUCH better.
If you KNOW things are going to be last minute, you work with your editor on this so it's not a big surprise to all concerned.