Impress R&D (summary translated from Japanese) released a report yesterday on the Japanese ebook market. They found that ebook sales rose 23.7% in 2009 to 57.4 billion yen from 46.4 billion yen. (from $527m to $652m). That also predicted sales for 2014, but I'm giving the predictions the attention they deserve (none).
If you're looking at the %23.7 and thinking it's low, you're looking at the wrong number. Look at the total sales. $652 million is nearly twice what the AAP reported for the US market for 2009. Japan had a bigger ebook market even though they have only 40% the population.
The %23.7 growth could have 2 causes. The Japanese ebook market might be more mature than the US one or the US growth could be attributed to all the media attention generated by the ereader hardware companies. Japan doesn't have the ereader market that we have in the US; most people read on their cell phone.