Can naphthalene, which came into contact with a set of books, cause harm or poisoning to my friend who will receive the books, or to people who will handle them later?

What you did to preserve the trust is a good deed for which you will be rewarded, and there is no blame upon you for the material scattering on the books because you did not intend that. If the material does not cause significant harm, then there is no need to inform your friend. However, if it causes real harm, then you are obligated to inform him.