Naturally I spoke too soon about the book formatting being done. Anyone familiar with what's been going on will not be surprised to learn that…(drumroll)…all this time, the book template had actually been a half-inch too tall! And all the random line breaks that were (for some reason I can't fathom) inserted and deleted are now showing up in all the wrong places, and the entire book is going to have to be re-re-re-re-proofed to catch it all.
The good news is that we asked for and got the editable file and are taking control of the process ourselves. This company doesn't (for some reason I can't fathom) normally provide that file to their clients, but anyone could see that this fiasco had to be put to a stop before anyone's capacity for stoic professionalism was exceeded. And that's all I'm going to say about that.
Except that if I had time for a side business, book formatting would actually be a pretty fun gig to get into.