Might also check out Barnes and Noble's Nook reader.
From what I understand, the Kindle requires you to purchase books only from Amazon, while with the Nook, you can purchase from numerous sources, including being able to check digital books out of many libraries! You can "check them out", they appear on your Nook, and two weeks later, they disappear (you return them).
Nook also has a "lend" feature, which allows you to lend books you've purchased and read to friends who also own a Nook. Again, two weeks after the date you lent the book to them, it disappears from THEIR Nook, and reappears on yours! (No need to remember to ask for your book back!)
Step into a local Barnes and Noble, and you can read ANY digital book they sell, while in the store, for up to one hour!
Nook has some advantages over the Kindle and Sony, and, of course, those have some advantages over the Nook. Read some
reviews of the three major book readers, then see which one fits your style best!