Open your book flat on the table. Measure the length from end to end. Add 2-3 inches for "ease". Then add 8-10 inches for the "pockets" at the ends for the covers. Write this number down on a piece of paper.
Now, close your book and measure it's height. From top edge to bottom edge. Add 1 inch. Write that number next to the first one. These numbers are going to be how long to make your book cover.
You are going to knit a scarf for your book in garter stitch. Cast on however many stitches it takes to get that 2nd measurement. (the one from top edge to bottom edge) Then, just knit every row until you have a scarf the same length as the 1st measurement. Bind off and weave in your ends.
Spread your book out again and also spread out your scarf on the table or floor below it. Fold each end in 4-5 inches to make a "pocket" for the front/back covers. Use your book to help you with this. Sew the pockets in place on the top/bottom edges and weave in the ends again.
VOILA!!!!!!!!! Your new book cover!