Possibly Keating's Insect Powder would answer as well or better than pepper.
Rats and mice will gnaw the backs of books to get at the glue, so, means should be taken to get rid of these vermin if they should appear. Mice especially will nibble vellum binding or the edges of vellum books that have become greasy with much handling.
Cockroaches are very troublesome in libraries, eating the bindings. Keating's Insect Powder will keep them away from books, but only so long as it is renewed at short intervals.
The Chairman of the Society of Arts Special Committee says on this point:--
"It is important that a just medium should be observed between the close and loose disposition of books in the shelves. Tight packing causes the pulling off of the tops of book-backs, injurious friction between their sides, and undue pressure, which tends to force off their backs. But books should not stand loosely on the shelves. They require support and moderate lateral pressure, otherwise the leaves are apt to open and admit dust, damp, and mildew. The weight of the leaves also in good-sized volumes loosely placed will often be found to be resting on the shelf, making the backs concave, and spoiling the shape and cohesion of the books.
"In libraries where classification is attempted there must be a certain number of partially filled shelves. The books in these should be kept in place by some such device as that in use in the British Museum, namely, a simple flat angle piece of galvanised iron, on the lower flange of which the end books rest, keeping it down, the upright flange keeping the books close and preventing them from spreading."
He also speaks of the danger to bindings of rough or badly-painted bookshelves:--
"Great care should be exercised when bookcases are painted or varnished that the surface should be left hard, smooth, and dry. Bindings, especially those of delicate texture, may be irreparably rubbed if brought in contact with rough or coarsely-painted surfaces, while the paint itself, years after its original application, is liable to come off upon the books, leaving indelible marks. In such cases pasteboard guards against the ends of the shelves are the only remedy."
It is a well-known fact that the leather of bindings that are much handled lasts very much better than that on books which remain untouched on the shelves. There is little doubt that the reason for this is that the slight amount of grease the leather receives from the hands nourishes it and keeps it flexible. A coating of glair or varnish is found to some extent to protect leather from adverse outside influences, but, unfortunately, both glair and varnish tend rather to harden leather than to keep it flexible, and they fail just where failure is most serious, that is at the joints. In opening and shutting, any coat of glair or varnish that has become hard will crack, and expose the leather of the joint and back. Flexibility is an essential quality in bookbinding leather, for as soon as the leather at the joint of a binding becomes stiff it breaks away when the boards are opened.