Maybe something like "The last sentence in Chapter 8" "Fourth sentence in chapter 9" and so on.

I still think GoodReads would have issues there.

The first thing Goodreads asks you to do when you create a new account is to list and rate a bunch of books you've read and either liked or hated. This is important, because they (ostensibly) want to recommend other books to you.

When I first created an account, I quickly rated maybe 50 books on a 1-5 scale I had read over the years. I didn't have most of them handy.