This is because if the chain were to be placed on the spine the book would suffer greater wear from the stress of moving it on and off the shelf. Because of the location of the chain attached to the book (via a ringlet) the books are housed with their spine facing away from the reader, with only the pages' fore-edges visible (that is, the 'wrong' way round to people accustomed to contemporary libraries). This is so that each book can be removed and opened without needing to be turned around, hence avoiding tangling its chain.