Must someone who purchased books in installments while underage, and whose sales contract is illegal, pay their bills, or should he be informed of the possibility of returning the books?

If you are a minor under legal interdiction (such as a child or a prodigal), your contract with the company to purchase books is not valid unless approved by your guardian. If your guardian does not authorize it, the contract is invalid, and they cannot demand payment from you; rather, they will reclaim the books. However, if your guardian approves, or if you are an adult, sane, and mature, not under interdiction, then the contract is valid, and the price of the books remains a debt for which you are liable until it is paid.