These are the main groups I'm talking about: (US books)
1923-1963, Copyright was renewed: 95 years after publication date
1964-1977: 95 years after publication date
(the rules get even more strange post 1977)
For the first group, the won't be public domain until 2018-2058
For the second group, its 2059-2072
And that is only for US published books. The rules vary widely by country.