This question has always bugged me. The interdict of Merlin stops spells being learnt from anything but a living mind. Yet Tom Riddle is specifically said to have learnt the horcrux spell from a book. Why are there even spells in books?
Who would believe the ritual for the philosopher's stone that is written down since no one can learn it from a book anyway?
The existence of books containing and describing spells seems completely illogical in that world.