I just finished a rather large chart comparing the prices and availability of ebooks at some of the major stores - Amazon, Sony, Barnes and Noble, BooksOnBoard, and FictionWise.
You can see the chart and full explanation here:
I was motivated to do this because of Sony and Barnes and Noble's insane claims regarding their book selections (and to a lesser extent Amazon's) given that they are including all of the public domain books available through Google - books available to pretty much anyone. Also, I wanted to see what Barnes and Noble was doing and this was the best way to get some answers.
I checked 28 books - the top two from each catagory from the NYT Best Seller lists, and then a group of my recent purchases and things on my wishlist that were not available on the Kindle when I placed them there. So it isn't the most scientific study in the world - but it covers a wide range of material and genres.
1 book was not available on any platform
Sony 23/28 (big improvement)
Barnes & Noble 13/28
Barnes and Noble really sucked here. So you can take a look at the graph as well as more analysis on my blog linked above.