Thanks for the answer but how can I understand it needs a preposition? I'm always confused about relative clause questions with prepositions. Are there any clues or techniques to find the correct preposition?
Why does shelf need on here? We can also say in the shelf or at the shelf ,right?
Think of the verb in a main clause - You find books on a shelf. That tells you that the preposition is necessary in a relative clause. At and in don't work.