"The Sonny Bono Memorial Collection"

There's an interesting little exception in American copyright law that is being exploited: if a book published between 1923 and 1941 is out of print, a physical copy can be scanned and released into the commons. And it's being actively exploited in libraries across the country!

Hence, the Sonny Bono Memorial Collection! (link)

This is a fascinating project by the Internet archive. There are currently 709 books in the collection, I have no idea at what rate new items are added. The one book that I looked at was viewable online and downloadable as a PDF, epub, Kindle, Abbey, and in some other formats.