Hi Cowboys fan-
First, let me wax eloquent on ebook readers in general. They have totally re-energized my reading regimen, and I can tell you that I've read more good books over the last year than I had in the previous five. They're very user-friendly and getting books, which was always the biggest hurdle for me due to time constraints, takes mere moments at home or on the fly.
Personally, I'd stick with one of the bigger competitors, for a couple of reasons. The most important is the size of the library. I have the Nook (non-color version) but I know I'd be just as happy with the Kindle. They look nearly identical, both hold more books than you'll need, and both have huge libraries available. These are the two I'd recommend you look at.
I also have the little 6" Sony reader, and I really like it a lot. I have to plug it into the computer, but frankly that's not something I find inconvenient or constraining. I actually like this device a bit better than the Nook - it's more spartan with just a few controls and none of the bells and whistles of the Nook. But it has a battery life that's forever, and there's less "gadget distraction" with this device than there is with the Nook. There was a time when the price difference between the Sony and the Nook/Kindle made the Sony worth considering; I don't think that's the case anymore, but it is a very nice little device.
As others have alluded to, I'd also avoid any ebook reader with backlighting. They have much shorter battery lives, and lead to eye strain. It's very easy to get a little dedicated light (actually, that Multi-Edge Light device that mo1950 mentioned sounds fantastic - I have a light, but I'm going to look into that). If your wife wants a websurfing device that also does double duty as an occasional ebook reader, that's one thing. But if she's looking for primarily an ebook reader, my advice is to get one of the dedicated ones.
Good luck and I hope she enjoys it as much I like the ones I have!
Chris