The person who requests something is called...?

Let's say that I have a collection of books, and someone requests one of them. How would that person would be called, in one word?

The only thing I have been able to come up is "pretender", but does not sound right, for some reason.

Answer

Yet another option would be applicant.

noun

1. a person who applies for or requests something; a candidate:
   an applicant for a position.

Reference:
http://dictionary.reference.com/browse/applicant