It's Google's version of Project Gutenberg, and it's separate from the bookstore. There is a link/tab to it in the Reader Library software from Sony, if you use that. You should be able to get in with your reader store login.
Tangent here: Several, if not all, of the classics are free in Google books and cost money in the Sony store. Never ever buy one of the classics from Sony, get it for free either from Google or Gutenberg instead.
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