On its Inside Google Books blog, Google offered a behind-the-scenes look at the Doodle. Here's how they picked out the books:
"... our Google Books editorial team curated a collection of free and featured Dickens classics available in the Google eBookstore in Dickens' native land (United Kingdom) and some Commonwealth countries (Canada, Australia) as well as the US -- a relatively new nation that Dickens himself visited in 1842 and 1867."
It's not as though Google hasn't used its homepage for promotions before. However, it hasn't used its Doodle in such a manner.
conversion percentage, too, this could be good data that Google could provide to advertisers, as well.