From O’Reilly radar:
At least as measured in terms of number of unique applications, Books have grown the fastest over the last 12 weeks. (Data for this post limited to apps on the U.S. iTunes store through 3/1/2009.)
Most of these books are free, but an increasing number are paid for, including many at over $10.
Across this and the Kindle 2 I think we are seeing the beginning of the end for dead tree books.