What is the ruling on not returning books borrowed from the mosque? Is it permissible to buy alternative books and place them in the mosque, and how can one atone for this sin?

Whoever takes books that are endowed (waqf) from a mosque must return them. If they are damaged or lost, he must replace them with others of the same kind, along with repenting and seeking forgiveness for violating the endowment. Sheikh Ibn Baz emphasized the impermissibility of taking Qurans or books from a mosque, because they were endowed for Muslims to benefit from them within the mosque. Whoever takes anything from them must return it, and if it is damaged or lost, he must replace it with something similar, along with repenting and seeking forgiveness.