Strictly speaking, book recommendation both fall under these two "out-of-topic questions" categories:
- They are similar to product recommendation, as they become obsolete relatively quickly,
- They request people opinion, instead of "facts, references, or specific expertise", as the Help page states: "You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page". There is no way to tell what could be the right answer to such a question as "Could you recommend some books?".