You bought this book, I trust, or you borrowed it from someone who did. If you stole it, you are a thief, whether you are caught or not.

What if I borrowed it from the library ?  I'm pretty sure it doesn't turn me into a thief.

I assume the library either bought the book or it was donated to the library from someone who bought it, so borrowing a book from the library is, in a sense, borrowing it from someone who bought it and, no, it doesn't make you a theif.