I enjoyed the whim to read '50' or '100' books in S3, but I still had trouble keeping track of which books they'd read. I'd have to keep all the unread books in their inventory and the read books in the bookshelf to keep them separate.
I've noticed (in S4) that some non-skill books don't have a little circle indicating progress while reading, so if your Sim has a whim to read a book or needs to read a book for an aspiration, it doesn't count. It seems to be a variety of genres, and the books for children don't count for adults, even if your Sim has the 'childish' trait.