Can a 14-year-old be taken to collections for a set of books delivered to him after purchasing one book?

No! 14 year olds, are not able to enter into any sort of contractual agreement, for anything. If you misrepresented yourself by signing the application, which usually says (18 or older). They might be able to hold your parents accountable for the debt. I seriously doubt it though. What probably happened is, it was a trial basis and you were supposed to tell the company if you wanted to continue purchasing books. Have one of your parents write to the company, explaining the situation. They might ask you to return the unpaid for books. Whatever, relax, you are not in trouble.