Life Lesson # 1. It's simple really... when you borrow something, return it after use.
It may be "just" a book, "just" a CD, "just" a shirt, or heck even "just" a pen but it's good manners and basically "just" overall polite to return something that you've borrowed because it may be "just" just to you, but it may not be "just" just to the owner. (note the "" on every just word)
I'm saying this based on experience and because I was going through my things and I am missing some books, DVDs (all original, thank you), and clothing. I don't know if those who borrowed from me just forgot to return it, they lost it, or they just don't want to return it anymore. I'm not mad. Just saying.
In case you're one of those "I was really planning to return it but I just keep on forgetting" type, life lesson #1 applies to generally everything that has something to do with borrowing. Borrowing means it belongs to someone else hence, you should return it.
I'll say it again, whatever it is you borrowed, have the courtesy to return it to who lent it to you. No matter how long it has been already. Oh please, especially if s/he is not your close friend. That would be embarrassing.