Interesting, although I have to say that I never felt the need to buy a Java book (not even Hibernate or Spring books), because the online documentation available is so good. I bought two Ruby books and a Groovy book because the online documentation available seems rather unsatisfactory. Perhaps that is just me, but if not, it would shed a different light on the book sales numbers.
Tim O'Reilly says that himself - he basically built his business around bad documentation. That reminds me, his Alpha Geek talk is exceptionally good.