Both Microsoft and Google have been scanning older books that have fallen into the public domain, as well as copyright-protected books under agreements with some publishers. I have used both sites in researching Louise Brooks, Frank Wedekind, silent film, and Jazz Age topics. And, I have found a bunch of stuff - including vintage reviews of Brooks' films in obscure trade journals like Educational Screen. I was also able to download a whole lot of early books on film. It's a pleasure to have Movies and Conduct, a book from 1933 about the effect of the movies on youth. (Louise Brooks is mentioned.)
Admittedly, I am an information junkie. Microsoft's discontinuation of this project is a sad thing.