Comment by watwut
There is no real way for teacher to check whether I read the book or not. People who read books regularly fail trivia tests and people who did not read them can quick read "about the book" analysis off web and call it a day.
And crutially, my inclination to finish assigned book and my willingness to read books in general are unrelated. A kid that reads a lot wont neceasary enjoy and finish assigned books - I know I skipped quite a lot of them.
If your premise is that there is no way to test knowledge, then I can see how you might not want to assign books to be read, but I think there are ways to test knowledge and that reading a book should result in knowledge of the book.