But I like this method of making covers as it creates a very clean edge:
It's not easy to describe and I've not got a formula but basically you sew the outer fabric (having decorated it to your taste) and lining together along the SHORT edges, right sides facing.
You then push the sewn edges in towards the middle of the cover until the fabric you see is the width of the open book plus a smidge for movement! Easier to do than describe, photograph or draw:
Once you've got this to the right size you sew the top and bottom edges together, either leaving a gap to turn right side out or slitting the lining fabric to do a bonded closure as per Melody Johnson. It's rather magical, as even now I can't quite see how it works!!!!