When I was a teenager I used to “steal” books from the library, too. It was mostly reference books on chemistry, physics and electrical engineering, where the alotted lending time of two weeks just wasn’t enough. One could return borrowed books after two weeks and ask for an extension, but I didn’t have a car and the library was a pretty far way away, so this wasn’t practical for me
When I moved out of town I returned all the books to the library and put them back on the shelves where they belonged 🙂