For textbooks, the only decent e-reader is going to be the Kindle DX. The extra "screen" size makes all the difference when it comes to reference texts. The iPad is could potentially be another solution if color is important to you, but since it has considerably more capabilities than a reading device, you have the extra cost and weight to consider.

I'm not knocking on the Nook (I have one, and I like it), but while it's fine for most reading, it doesn't work well for texts that I need to refer back to frequently or "learn" from. The Kindle DX and the iPad have proven themselves (to me) to be much better for those sorts of things.