The life of a book editor must be a difficult one. The frustration in the redlining is overwhelmingly palpable. I can't imagine how it must feel to have to read this entire book and comment on everything all the way until the end. Even for money, some things just aren't worth it.
Those of us who have edited screenplays for friends, for acquaintances, for business contacts and for money recognized every.single.line. What I usually do? I write out my line-by-line gripes and if they're useful, I include them. If they're harsh enough that I think they might crush the person on the receiving end, I don't. Either way, I give them a nice 1500-2000 word summary of the strengths and weaknesses. But if you're trying to whip a manuscript into shape, the line-by-line shit is all that matters.