Sure, "book" and "guy" may be improper nouns. "Guys" ('guy' plural) may also be an improper noun but when used to refer to the group of people you're speaking towards (as in "guys I'm trapped in the elevator"), it's used as a pronoun.

It could also be argued that "guys" used in that context isn't a noun or pronoun at all but rather just a filler word as it doesn't communicate any meaning (you are implicitly talking to the people you are talking towards).