A big difference between Google Print and libraries is that physical libraries have to buy a copy of the book they loan out. The American Library Association states there are 117,664 libraries in the US, so you could sell thousands of copies of your books just to libaries. Of course, over 93,000 of them are school libaries, so if you don't have a book they'd want to stock you're out of luck.

I think Google might be better off if they took the approach they're using with Google Scholar for copyrighted books.