Hello and welcome! I am new too! In US books go into public domain 80 years after the death of the author. The original intend of copyrights was to reward the author, but I am not sure how having a copyright in afterlife would help the author. It hurts the innovation and creativity but big corporations love it!
Anyway, you can find great public domain books at
http://www.gutenberg.org/. There is a lot of other books available which are not in public domain (through torrents, etc.), but it is illegal and you
should not do it because super rich owners of giant corporations will have a less money.