I could be wrong but I don't think being an ebook in the US is a requirement this month.
If that's the case, I will also second (or third if it's already seconded) The Spy Who Came In From The Cold. I'm intrigued that it's on those lists of great books.
If it's the third in a series, will we be lost if we haven't read the other two though? Or will it be easy enough to pick up?