Um, is there anyway around this legality? Or could I face jail?

It's hard to guess what will happen if you do nothing. It's not a good idea anyway. If you have the book return it. If you do not have the book, left the library know. Then you're just looking at paying some money. It's possible that taking the book and appropriating it to your own use, can constitute theft. That might be the worst case scenario. It's one to avoid.