As always, I strongly encourage you to get a copy at your local independent bookstore, but it's also available at Barnes & Noble and on Amazon.
If you want a signed and/or personalized copy, you should order from WORD bookstore in Greenpoint, Brooklyn. They'll let me know and I'll head over to sign a copy just for you! (If you only see the hardcover listed, just click "other editions of this title" to get to the paperback.)
If you've already read The Princess of Trelian and are waiting for book 3 ... I can tell you that the third book is now scheduled for fall of 2015. I'll post more specific details when I have them! :)