Measure your book. You need the height, plus the width when open (make sure to include the spine).
Draw out a rectangle the size of your book, plus 2cm seam allowance (so if your book measured 6 inches by 4 inches, you'd make a rectangle 8 inches by 6 inches).
Cut one from your pretty fabric, and one from your lining (if you're using a lining - you don't have to).
On your pretty fabric, also cut out two more rectangles measuring three inches by your book height plus 2cm seam allowance (so using our example above, 3 inches by 6 inches).
At this point, hem everything (should take up 1cm of your seam allowance). If you're using a lining, place it back to back with the big rectangle, them hem the pretty fabric over it so it is encased.
Layer your fabrics as so:
- Lining face down (if using - it'll be sewn into the big rectangle anyway!).
- Big rectangle face up.
- Small rectangles face down.
Sewn where indicated in purple (so round the outside edges of the little rectangles). For a really secure edge using a sewing machine, sew round with a running stitch first, trim the excess to about 2mm, then sew over the edges with a zigzag stitch. If you're hand sewing, use a back stitch.
Fold your small rectangles back on themselves so they over lap the lining/ back of the big rectangle if no lining was used. Everything should now be right side out.
Decorate your book cover however you like. Add a strap to keep it closed, add lace trim, appliqué or paint on images and text, bead it, embroider it... it's really up to you! I used to sew pockets onto the small rectangles too to keep pens in.
The book should slip into the flaps created by the small rectangles and stay there securely. If not, you may need to make the smaller rectangles slightly wider - try 4 inches instead of 3 inches.