AAP's stats for the overall book market showed a 6% growth in 2012 (from $6.7 billion to $7.1 billion), with most segments except hardcover reporting positive growth. Adult and religious hardcover market segments both shrank in 2012 by several percent, a sign that the more expensive editions are a luxury that consumers can do without in this tight economy.
The combined ebook market was up 41% (from $1.1 billion to $1.54 billion), with children's ebooks showing the best growth (120%). That puts ebooks at about 21.67% of the total US book market, just under the combined paperback market segments (trade, massmarket, etc) which the AAP pegs at $1.66 billion. Though I am not sure that figure is reflective of the real paperback market; it's not clear where YA paperbacks are factored in.
Update: The AAP is reporting that ebook market share was 22.55% in 2012. The reason my number differs is that their figure is based on stats that were edited slightly so the 22.55% reflected the same data set as collected by the AAP in earlier calendar years.
All in all, the growth rate of the US ebook market does appear to have slowed considerably. The AAP reported in early 2011 that the calendar year 2010 saw a 165% increase in ebook sales, and last year the AAP reported that 2011 ebook sales were up 117% over that of 2010 (I still have the emails).
Of course, these figures don't necessarily reflect the entirety of the US ebook market, just the sales data reported to the AAP. As anyone who follows this topic should know the AAP figures only include data provided by 1,193 AAP members.
It is entirely possible that the growth of indie ebook sales now outpaces the rate reported by the AAP, and there is even some data to support that conclusion. Amazon, for example, reported a much higher growth rate for their ebook sales in 2012 - 70%, in fact.
And as a reader has just reminded me, B&N reported earlier this week that a quarter of sales in the Nook Store are self-published ebooks and thus not reflected in AAP figures. Apply that percent across the market and there could be quite a lot of ebook dollars that aren't being counted by the AAP.