Measure the height of the paperback book. Measure the width of the book's front cover and the width of the spine. Write down your measurements for later reference.
Add 1-1/8 inches to the height measurement from Step 1. Multiply the cover width by two, add the spine width, and then add 1-1/4 inches. Cut your book cover fabric into a rectangle matching the height and width totals.
Add 1-1/8 inches to the Step 1 height measurement. Subtract 1 inch from the Step 1 front cover width measurement. Cut two pieces of fabric to those measurements.
Cut two strips of 1/4-inch, double-fold bias tape -- in a colour that complements your book cover fabric -- to the total width measurement from Step 2. Cut two additional bias tape strips to the total height measurement from Step 2.
Insert one of the largest fabric rectangle's long ends into the fold of one of the long bias-tape strips. Slide the cut edge of the fabric as far into the fold as you can without wrinkling it, and then pin the bias tape in place. Place the fabric right side up on your sewing machine. Sew the bias tape to the fabric using a 3/16-inch seam allowance, and then remove the pins. Repeat with the fabric rectangle's other long end and the second long piece of bias tape.
Pin and sew one short bias tape strip to one long end of each small fabric rectangle, using the method from Step 5. Remove the pins.
Lay the large fabric rectangle right side up on a flat surface. Turn one of the small fabric rectangles right side down. Align the long end of the small fabric rectangle with one of the short ends of the larger rectangle, and pin the fabrics together. Repeat with the second small fabric rectangle and the large fabric rectangle's other short end.
Machine stitch the tops, bottoms and outer edges of the small fabric rectangles to the large fabric rectangle, using a 1/2-inch seam allowance. Leave the inner edges of both small fabric rectangles unsewn. Remove the pins, and turn the book cover right side out.
Cut thin, lightweight cardboard into two rectangles, each measuring the book height from Step 1 by 1/4 inch less than the front cover width from Step 1. Insert one cardboard rectangle into each book jacket pocket.
Insert the paperback's front cover into the book cover's left pocket, in front of the cardboard insert. Repeat with the paperback's back cover and the book cover's right pocket.