Based on a previous design of mine, I have just completed the Book Bag!
I added a gusset all round and some purchased handles.
The idea came from when I was coming out of the Maison Bleue with a book in my hand, and thought how convenient it would be to have a special bag for this purpose.
I haven't lined the bag as I felt it was unnecessary for this purpose, as there were no small items to fall through. If the bag were to be used for other things then, of course, a lining would be 'de rigeur'.