The ones you can lend to your friends after having read them. The ones that your parents have read, kept and that you took in their bookshelf when you were a kid.
The ones that your friends gave back to you and that will wait in your bookshelf for your children to take and read. The ones their friends will borrow and that will probably giggle a bit, because they will have difficulties deciphering the notes your father wrote on a margin.
Because if content has a price, knowledge ought to be free.