I just wanted to give a shout out to some great resources that I have seen lately.
First the Make A Bag series at ikatbag. Holy Uncontrollable Detail, Batman! This series of post goes into amazing detail on bag shapes, straps, finishing and linings with more posts on the way. It is not a series of tutorials, but an in depth discussion on bag theory so that you will be able to design and make your own bags. You will want to keep reading even after the series is over to see the incredible things she makes for her kids from cardboard. The woman is amazing! And a hoot to boot.