When the Association of American Publishers (AAP) released its sales figures for the month of February, the headlines were easy to compose: e-books have surpassed print in all trade categories.

E-books have become the format-of-choice, these figures suggest. In January, the AAP said that e-book sales were up 116% year-over-year, and for the month of February that growth accelerated even further. February 2011 sales were up 202.3% from the same time last year.

Audiobook sales have also continued to grow. They were up 37% year-over-year for February, following an increase for the January period as well. That increase, along with the rise in e-books, point the change in our book consumption habits: clearly we are “reading” (or listening) on-the-go – in the car, on a mobile device.

Of course, a trade paperback has always been a fairly mobile “device” on its own accord. But it’s size and weigh hasn’t saved it from falling sales. E-books became the number one format in all categories of trade publishing in February, surpassing adult hardcover, adult paperback, adult mass market, children’s and young adult hardcover and children’s and young adult paperback.

Is This a Post-Holiday Sales Trend or Something Longer Lasting?

The AAP suggests that this surge is a continuation of post-holiday sales, as people buy e-books to load onto the e-readers they received as gifts. Whether or not this trend will continue long after the novelty of new tech toys wears off remains to be seen.

What’s also worth watching: not simply the sales of new titles, but the renewed consumer interest in old titles, backlisted books that have been in print for over a year but that people want to buy, again, to load onto their new e-readers.

While that excitement to buy books might sound like good news for the publishing industry, the buzz over e-books hasn’t stopped sales overall from falling. For the year-to-date, sales of e-books have grown by almost 170% to $164 million. But the sale of print books, which is still a far larger portion of overall publishing revenue, has fallen by almost 25% to $442 million.

Tom Allen, the CEO of AAP, puts a positive spin on the news: “people love books.” Perhaps.

And perhaps e-readers will spur a new passion for reading (and buying). But for many readers, it may be less that we’re buying more books, but that we’re buying books in a new format, taking away from the revenue from the sale of $25 hardcovers that have long floated the industry and now purchasing our books in $10 digital formats. That means the publishing industry has to sell a lot more e-books to make up that difference.