It is generally not legal to rip CDs borrowed from a public library. Libraries typically have a lending agreement that you enter into when borrowing physical materials, often outlined in your library cardholder contract or in the library’s physical materials use policies. These usually indicate that the materials are for personal use only and may not be reproduced, copied, or transferred to any other medium. This also applies to CDs. However, different jurisdictions may have different laws regarding this, so it's always best to refer to the specific lending agreement or policies of the library where you borrowed the CD. Some libraries may allow ripping or copying of media for personal use with certain restrictions, while others may strictly forbid it. If you are unsure, it's best to consult with a librarian before ripping CDs borrowed from a public library.