On works prior to 1978, the copyright expires 28 years after the author's death unless renewed for another term. The author of the book ain't dead yet.
The net affect of this is about half of Edgar Rice Burroughs and H.P. Lovecraft's books have entered the public domain and they died a long time ago. That is to say very roughly speaking that books written in the 1930's and 40's are the ones entering into public domian these days. Tolkein's books will enter the public domain in 2030 - 2040 something.