Penguin has reported that e-book sales from the first four months of
2008 have surpassed the house’s total e-book sales for all of last
year. According to the publisher, the spike is "more than five times
the overall growth in sales, year-on-year, through April 2008." Penguin
Group CEO David Shanks said he attributed the jump, in large part, to
the growing popularity of e-book readers.
Addendum: Here is more on the economics of Kindle.