I often find myself buying books I want to read in the future simply because it's only $10-$25 and I don't want to forget about them (I never liked the wish lists for whatever reason). Which is easy to do since there's no penalty in space (either disk or physical)
I also find myself buying more from Amazon because I don't have to wait for shipping (and Kindle delivery actually makes getting the book faster than if I drove to the local Barnes and Noble)
So for me the Kindle has shifted my book purchasing to e-books but it has also dramatically increased the amount I spend on books from Amazon.