Have you discovered where you left your books?
where you left your books is a prepositional noun clause.
The clause is used as OP - object of a preposition.
The preposition is where and the object is your books.
Prepositions often begin prepositional phrases. The preposition teams up with a noun, pronoun or gerund as object of the preposition.
To find the object of the preposition, find the preposition and then put the preposition in the blank and ask '...... who or what ?'