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.
January 1, 2019 is (finally) Public Domain Day: Works from 1923 are open to all! is from Duke.
Public Domain Day Is Finally Here!: Copyrighted Works Have Entered the Public Domain Today for the First Time in 21 Years is from Open Culture.
Also, the Hathi Trust has a catalog of the new works that are in the public domain.