As Smithsonian Magazine wrote last week:
At midnight on New Year’s Eve, all works first published in the United States in 1923 will enter the public domain. It has been 21 years since the last mass expiration of copyright in the U.S.
Here are the three most useful resources about it:
How to Download the Books That Just Entered the Public Domain is from Motherboard.
Also, the Hathi Trust has a catalog of the new works that are in the public domain.