Wired has a buying guide looking at e-readers as possible Christmas gifts. While it is not super-detailed, it does provide a decent grounding in the basic features and issues associated with e-book readers and even discusses alternatives to full-fledged reading devices such as the iPhone. There are a couple of errors, though—as commenters note, the article refers to Sony’s PRS-700 which has actually been replaced by the PRS-600.
For those wanting more details, MobileRead’s reader comparison matrix makes a good starting point.