How long is book copyright protected according to the US copyright office?
Until USCO decides a marker. For example everything released
before 1923 is public domain in the U.S., unless renewed for
copyright, because along came audio. They added another marker to
somewhere around the late '60s, because along came color TV. They
had to renew copyrights, particularly Disney. These markers were
decided later.
Unpublished finished works are public domain 70 years after the
author's death.