Place the book jacket face down on a firm hard surface, assembling the pieces like a jigsaw puzzle if some parts have torn off. Use the bone folder to gently rub out and smooth creases. Repair pieces that have torn off of the jacket by carefully taping together the pieces, applying tape only to the back of the jacket. If some of the creases are severe and it appears they may easily tear off, secure them with a piece of tape as a preventative measure.
Turn the jacket face up. Apply a very small amount of rubbing alcohol to a cotton swab and test a corner of the jacket to see if the ink will run when rubbing alcohol is applied. If the ink comes off on the swab, you should clean dirt and grime using an art gum eraser, however alcohol works better when possible.
Clean clumps of dirt and grime with cotton swabs and alcohol, always using gentle pressure and patience, as book jacket paper is often quite delicate. If the entire jacket is covered with grease, nicotine residue or other dirt and grime, you can apply alcohol to a soft cloth and gently rub the jacket. Always ring as much liquid from the cloth as possible and never use water to clean book jackets.
Position the plastic book jacket cover, and your repair is done. The best book jacket covers are open edge adjustable covers and are widely available in stationary or book stores. Jacket covers come in a variety of weights from 1 to 2 millimeters. Books that will be stored can be safely placed in lighter weight jackets, but if the book will be in heavy use, it is best to use the heaviest weight cover.