Here's the ereader matrix for "large" devices
. I'm afraid I haven't used any of these, so I can't make recommendations. Anything that has eInk technology will have good battery life, and Pearl eInk has a very crisp contrast.
Since you're scanning textbooks, I'd really recommend something that's able to handle the page size (which you realize...). I imagine many of the texts will have tables and graphics, so it'd be ideal to not have to strip out any of that.
EDIT: after looking through the Matrix, I
really want the Pocketbook 902 (it's the 903 without 3G or touchscreen)