We are weekly library visitors and each of us having our own book bags works for us. It limits the number of their books I have to carry and (mostly) restricts them to the ones they can fit. Auntie(s) Katie and Charlotte made awesome bags for the boys a few years ago but they've finally given up. So I went though my fabric bits and pieces and sewed up these easy tote bags (the same pattern as my one) adding a little pocket for their library cards. I would have used zips but I haven't faced that sewing fear yet... I keep promising myself I will.
I've told the boys to put the books back in their bags when they've read them so that we know which ones are ready to go back. So far so good.