Amazon : Search Inside
Recently I have been buying some books from Amazon and I think the Search Inside feature totally rocks ! Especially in a place like Amazon that shows related and similar books (which is again quite accurate), it is easy to get confused while choosing. Moreover often the titles of the books and reader reviews help little in the process of choosing since every decent enough book will have somebody giving it raving review and somebody else calling it names.
With search inside feature however, you can actually see the contents and read few pages. This makes the whole process much more personalize and much closer to the experience of buying books from an actual store. I believe that those not providing this facility for their books, stand to loose in the long run.