In the US, you can date public domain for the most part by the date of "Steamboat Willie," the first Mickey Mouse cartoon. Any time it gets close to falling into public domain, Disney opens up Scrooge McDuck's vault and buys enough votes in Congress to prevent it. There's a small loophole that was closed in the 70's which says works which didn't have their term renewed (28 years) fall into the public domain, but it's dicey to figure out whether it applies.
Google is no authority for this. They basically announced with the opening of their Google Books site that they didn't care whether the book was copyrighted, they'd only take it down if the author reached for a lawyer. So if it's on Google, that means pretty much nothing at all about whether the book is still under copyright.
The most reliable date is 1928. Books before that are probably Public Domain. But no guarantees.