When I look at these stats it tells me a couple things (domestically, not internationally). Now, remember these are my own conclusions. You read the article and let me know your ideas on where the industry is trending.
In regards to the overall book sale declines, this could just be an indication of the Christmas spending period ending in December. It's not a big percentage and ties in the the national spending dip in January every year.
I do have a little concern with the Adult Hardcover, but since the paperbacks and e-books are increasing my thoughts are purely economic. People won't or can't afford to buy the elaborate hardbound books and are opting for the cheaper version. It's the same story, just different medium.
The Children's/YA hardcovers and paper backs shows a dramatic trend away from traditional reading methods. My inclination is that these younger readers are part of the mass increase in e-books and are reading from their generation-specific technologies.
And moving to that amazing percentage in the e-books category...WOW! I think this is the biggest alert for the industry of the direction reading is going. Just like the last days of the CDs. People were in denial about the MP3 and Ipod's...but it came and it came quick. Now look at the CD sales.
Paper books are just on the end of their life cycle. It's not bad. It's not good. It just is. We have an eco-friendly society that is on the technology advancing bandwagon. The good thing is reading is not going with the paper formats. It's still there, just different.
I'm going to keep an eye on these figures. It plays a lot into my publishing goals, marketing/promotion strategies, and target market objectives.
Good writing all! (No matter what format you work ends up in.)