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.

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

"Requester" has the meaning your looking for.

Do note though that this is a word that most native English speakers would never use as we would instead say the likes of "The person requesting the books".

However, the word is easy for English speakers to understand and if you want a variable in your code to refer to such a person then "Requester" is the word to use in my opinion.