Okay, here are photos of my Overland Donner Bag. I love it, because it can hold two bottles of water, both the Birnbaum's Guide and the UG with room to spare (I don't really need to carry them, I'm just trying to give you a sense of size), has a space for my digital camera or a cell phone, slots for pens, compartment for cash, etc. And the weight is well distributed.
My favorite part of the bag is the compartments for water. Not only are there two compartments, but they are located and attached to the straps/sides of the bag, which helps distribute the weight. Plus, they don't get all stretched out like water holders made of mesh. The pictures you see don't show my bag fully loaded; they just contain the books and the water bottles. The bags will hold a lot more, including snacks, a water poncho, sunscreen, etc.
You can buy the Donner for about $52 in many places. Ebags is having a small sale on them right now, but you can get them through REI, ebay, or several other online retailers. It's kind of pricey for a Disney-only bag, but I use mine for all sorts of casual travel on the weekends, on day trips, etc.
I hope these pictures help!