A member of the Google Book Search team responded to my most recent post on Plain Label Books (The Mystery of Plain Label Books Solved!).
Google didn't purchase any titles from Plain Label Books, as I had speculated. Those books were included as part of their Partner Program. Anyone who has the rights to a book can submit it to Google Books for free. Several different versions of books in the public domain can be found there--some of them from Plain Label, others from Google Books' library partners.
There you have it, folks. Google didn't buy the Plain Label CD, it was Plain Label that signed up as a partner. Why pay when people come knocking on your door.
I wrote about Plain Label Books and its relationship with Google because so many of their books came up in the search engine, and I keep getting hits on those posts from people around the world who obviously saw Plain Label Books on Google and are wondering who they are.