The Entropy of Everything went live in paperback on Amazon earlier today for those who were waiting for a hardcopy. If you prefer to get your books through Barnes and Noble, it should be available there pretty much any minute--we had to do a little adjusting of the barcode on the back cover (it's always the little technical details, isn't it?).
Nook, Kobo, and Apple iBook are also in process--I'll keep you posted and let you know as soon as it goes live in those formats, too.
Here's a direct link to both the Kindle and Amazon paperback versions. Links soon to follow for all other formats!