N.L.
There are lots of choices out there, so you'll have to narrow down the field a little bit. Where does she normally get books? Obviously Amazon = Kindle, B&N = Nook. If she gets books from the Library, both Sony and Kindle have ebook checkouts for libraries (although Kindle books for libraries are newer). Does she want it exclusively for reading or does she want apps too? Does she prefer to read off of e-ink or is an LCD screen ok?Does she have to have touch screen or are buttons ok? There are good readers available for less than $99...it just all depends on what you want.
My personal preference is the Sony Touch. I had an older PRS-505 and loved it. I don't like that the Kindle locks you into their format (and I don't like the ads that come up on the lower models). I like the fact that it has both the touch screen and buttons. I also wanted a reader with e-ink..solely for reading, no apps.
However, I know lots of people that love their Kindles/Nooks. It just really all depends on what your preferences are.
Hope I at least gave you a starting point!