When I used to run RPGA games at the local gaming shop, I had to bring a number of books, a collection of miniatures, battlemat, maps, modules, and all the various handouts/paperwork for new members, plus enough pencils and dice to share. The local RPGA is very disorganized, so I often didn't know which of several modules I was running until the moment, so I had to be prepared for anything.
I bought a rolling toolbox. I got it at Wal-Mart on sale for about $20. It's heavy plastic, has a top compartment that holds miniatures and map tiles, a tool tray for pens and such, and a bottom compartment that is big enough for books to stand on their long end so I can read the spines. The bottom compartment latches to the top. There is an extendable handle so you can pull it behind you like a rolling suitcase. There are also several small containers built in which I use for a variety of thing.
I would sit it next to me and put the top compartment on a chair so that I could access both sections.
Now, I will say that the $20 Wal-Mart model is a little less sturdy than I would like. It takes some effort to get the two pieces attached when they're full of stuff. The heavier duty ones are double the price or more, unfortunately. They also make more advanced ones where you can access the top and bottom without separating them, but those are usually metal and much more expensive.
I found it a great solution. I used it weekly. It was of great help and was far easier than hauling a bag full of stuff or carrying a milk crate, which is what I used to use.
My two biggest beefs with it are that the handle is not long enough for someone of my height and that at home I have to keep it in the corner because the books will shift and throw it off balance, causing it to tip over. The latter is not a problem when all of the books are in there, just when only some are still in there.
I did a search trying to find the model I have. I could not. This links to a similar model, except that mine doesn't have whatever that yellow fold-out part on the bottom front is.
[ame=http://www.amazon.com/Stanley-Consumer-Storage-018602R-Mobile/dp/B000Q5GORC/ref=pd_sbs_t_njs_5]Amazon.com: Stanley Consumer Storage 018602R Mobile Work Center w/Soft Front Pocket: Home Improvement[/ame]
For my normal gaming, where I have more control, I generally either carry a satchel or a milk crate, depending on how much stuff I need. (Shadowrun is book heavy, for instance.) Lately, I've been working with systems where I have PDFs of all of the books, so then I can cut down to the core book and a laptop, or just the laptop.