This is the first expansion in nearly 6 months, and brings the total number of countries to 70.
Update: Or maybe not. For some unknown reason Google has removed the 5 countries from the supported countries list. There are now 65 countries listed, and not 70.
If you live in one of the 5 countries, you can now buy ebooks from Google Play and either read them in the official app or download them with Adobe DE and transfer them to the Epub compatible ereader of your choice.
You also have the option of uploading your own PDF and Epub files to Google, and then downloading them and reading them in the official Google Play apps for iPad, Android, and iPhone (or in your web browser). Here's how.
By my estimate, Google Play Books is one of the top 3 or 4 retail bookstores in the world, falling somewhere behind Kindle and iBooks and ahead of Nook, but that is just an estimate. Alas, no one is sharing actual revenue figures, so it's the best we have.