The Libraries proposal had a significant amount of activity early on, but the activity chart has been more or less flat for a long time. Many people who signed up are not active on the Stack Exchange network (and interest from existing users appears to be low) so the percentage is still very low.
The slightly newer Books proposal has a much more active looking activity chart, but still has a relatively low number of total commitments.
The Libraries proposal was created with the intention of bringing in the community from Unshelved Answers and that site has a large number of questions that would appear to be on topic with the Books proposal. In particular, the most popular tag is questions looking for a book title based on descriptions of plot, the design of the book, etc. (An example question similar to that was 6th on the Books on-topic list with 10 votes.)
Several of the top on-topic questions on the Books proposal appear to be within the category of the Library service known as reader's advisory which would certainly be on-topic on a Library answer site.
Of course, the Libraries proposal covers other topics related to library operations and librarianship that would likely not be on-topic with the Books site. On the other hand, comparing the off-topic questions on both proposals I'm not seeing any that would be on-topic on either proposal.