I made this for an old book that was falling apart to make it look nicer. The cover is knitted, but there are crochet edges on both sides, which join the flaps to the top. If you don’t like crocheting, you can also sew it.
If your book is another size, please knit a gauge to see how many stitches you will need. Also knit a gauge if you are using another yarn weight. Fingering works fine for a book, anything thicker might be an ill-treatment of your book. If you love your book, don’t use bulky yarn.