I wish there were compression bags which compressed stuff flat like a book for the specific purpose of jamming soft goods into book bags.
By the way, I have 2 vacuum zip lock bags in my messenger bag with small soft things compressed really flat. It's a new brand, which I've recently discovered at my local SafeWay: "Reynolds Handi-Vac". They have a gallon and a quart sized bags available.
http://www.reynoldspkg.com/reynoldskitchens/handi_vac/en/home.aspQuart sized bags holds the vacuum really well, but after about 1 month of EDCing I had to deflate them one more time.