Discarding or throwing your old books is not a good thing. The best way to utilise them is that you can give your old books to someone needy like donating in an orphanage or a school running for poor children only as this can help them a lot as many of them are not able to afford the study material. After your examinations just make a stack of your old books at one place and whenever you get time donate them and if some are of use then always prefer to keep it with yourself. So always try to make a wise decision. So suggest the same to other students too.