I agree with pinkiris - don't make the bag so large that she won't be able to lift it. If she's like me, the larger the bag the more stuff I can put into it and then give myself shoulder problems. For books I would use outdoor fabric or even nice upholstery fabric. For straps I would use a 5" strip of fabric, 5" fusible batting on top of that, then fold the strap in on itself, meeting the 2 sides in the middle, press well, then fold in half, press well, then sew at least one row of stitching on each side, and I sometimes add 2 more in the middle. I've used this for several bags for myself and I tend to put a lot into my bags - the straps are very sturdy. I would also attach the straps stitching in a square, then an "X" to make it even more sturdy. For the bottom of the bag I like to use a piece of plastic canvas cut the size of the bottom, then add a layer of batting, make a slip cover over the batting and sew around the edge. I've made over 20 bags using this method and my machine has no problem sewing over the plastic canvas. You might also consider an outside and inside pocket for little stuff, like keys. Sorry for being long winded. Hope this helps.