If she does a 'lot' of reading, as you have mentioned she does, I would suggest an e-ink model. It won't have the cool factor of the Fire which is more like a tablet PC but it will be a lot easier on the eyes than a backlit LCD screen.

E-ink is pretty much identical to real paper as far as the eyes are concerned. LCD is fine for light reading but not as easy on the eyes for longer durations. Some people say it doesn't bother them but there has to be a relative difference, even if you are lucky enough not to notice it.

With great power comes Awesome irresponsibility.