Interesting data from Flurry (a smartphone usage statistics service) suggests that eBooks were 10% of what is in the iPhone app store back in July 2009, but had reached 20% by October 2009. Over the same period, the proportion of games declined slightly (17% in July 13% in October) . The data are published in the October 2009 article of Flurry's "Smartphone Industry Pulse" series
These data don't tell us about whether the books are being downloaded, nor whether they are being purchased (as opposed to being free downloads). Also, of course, there are many other places you can get books for your iphone (e.g. by using the free Stanza Reader). Nonetheless, asFlurry say "We take the spike in demand we noted from an earlier analysis, combined with the flood of supply into the market as an early indicator that iPhone as an eReader is real. "