What is the ruling on taking a book in exchange for a book and then returning it, and what is the ruling on money taken for mediating the loan of books with the knowledge of the book's owner?

Exchanging a book for another for reading and returning it falls under the category of exchanging usufructs for their like, which is permissible provided the rental period is specified. As for your work as a broker or intermediary between book owners, or as a trustee for their deposits, you may take a fee for any of these services.