Cory Doctorow just posted this on BoingBoing:
An obscure copyright law is letting the Internet Archive distribute books published 1923-1941
The Internet Archive is calling this the
Sonny Bono Memorial Collection in honor of Sonny Bono's attempt to enact perpetual copyright law. He also worked to create "orphaned" works by placing public domain books back into copyright status with no clear copyright holder.
There is a loophole that allows libraries to make and lend copies of out-of-print books from 1923-1941.