Google is now offering free downloadable versions of public domain books that you can find in Google Book Search.
Unfortunately, there's no way to browse through a directory of books that are available. However, you can keyword search for them easily, sort of.
On the Google Book Search home page, you'll see two options below the search box. By default, "All books" is filled in. Change this to "Full view books." Now search for something you are interested in, Dante's Inferno, an example both the AP and Bloomberg cite in stories about the new feature.
When the results appear, click on one of the books that comes up, such as this one. Over on the right-hand side, in the column just below the title, you should see a "Download" button. That will let you download the book in PDF format.