Considering devices like the Kindle and Kindle on the iPod Touch can allow you to carry a large amount of reference (or other) material with you, pretty much full time, I wonder what material people think is valuable?

I understand the limitations of electronic devices, but in reality, with a bit of planning, electronic devices can be viable for a fair amount of time and you simply are not going to be lugging an encyclopedia set around with you with traditional medium.

Anyone else do this? What materials do you carry? What would you like to carry?

-john