I would also keep mind Sturgeon's Law ("90% of everything is crud", first published 1957 in his book review column, interestingly enough).

Most books are crap. Interchangeable crap; maybe you'd enjoy some of them or learn something, but you could pick at random from the interchangeable pile of crap and be equally entertained and educated.

If you trawled that 80% of pre-1964 books, most of it would be crap, simply because most books are crap, whether they were published in 1964, 2014, or 1864. And more of it would be crap, because if you are differentially renewing copyrights on books in your catalogue, you are trying to renew the good stuff and forget the crap.

That's not to say that good books didn't fall out of copyright, for a million reasons, but if you have five books you like from before 1964, and you go check the copyright status, don't be surprised if fewer than 4 are part of that 80%.