Kevin Brockmeier said this quote

[I]f he had to guess, he would say that the reason he doesn't want to loan the book out, to Ethan or anyone else, is because of the part of his personality that is one gigantic record-keeping system, a complex sifting and filing scheme that dictates what goes here and what goes there, turning his life into so many marks on a tablet. His mind would busy itself with the book's whereabouts every second it was away. He knows it would.