I understand your need to carry home your books and back to school again; maybe its this idea that you might just get some work done if you have books in your bag (well, that's me, anyway). Also, i like working with books in front of me instead of a PDF: it's just easier on the eyes, you can highlight, and most importantly Facebook isn't on the other tab.
Handy hints:
1. Get a two shoulder strap bag (not those messenger bags). It has to fit nicely just above the small of your back. Make sure the shoulder straps aren't too loose or low or it'll cause back pain. Make sure it's no more than 35Litres. I have one of these:
http://www.blackwolf.com.au/lifestyle/sidewinder/ , but maybe one of these will do too:
http://www.blackwolf.com.au/education/Larger bag = greater temptation to put in more books.
2. Think ahead - if you think that the next day there will he a mountain load of books, then take some home today.
3. Get another smaller bag and put some books in there.
4. Use PDFs for larger textbooks like your methods stuff.
5. Use your school spares and do that work!