The books are reproducing!! You should have opened it- maybe there was a tiny Stephanie Meyers book in there or something.
Was it one of those pregnancy eggs that teens are supposed to look after and care for, do you suppose, or just part of someone's lunch?If the former, I'd say they're jumping as the "librarian as babysitter" bandwagon a bit too early.
just be glad it was intact....
It was probably one of those alien creatures from that one episode of Buffy.