When a question starts with "when, who, why, where, what, etc" and the verb is associated with an adverb or preposition, it is natural to put those parts of speech at the end of the sentence in modern English.
What is the book about?
What type of book are you looking for?
Who did you buy the book for?
When a question begins with an interrogative like what, where, why, etc., it is normal/natural to put the preposition at the end of the sentence. Placing the preposition at the beginning of the question can sound either very formal or be incorrect, depending on the context. The link has examples.