The Dark Half

In 1985, 39-year-old
Stephen King announced in public that his pseudonymous alter ego,
Richard Bachman, was dead. (Never mind that he revived him years later
to write The Regulators.)
At the beginning of The Dark Half (1989), 39-year-old writer Thad Beaumont announces in public that his own pseudonym, George Stark, is dead.
Now, King didn't want to jettison the Bachman novel, titled Machine Dreams, that was he working on. So he incorporated it in The Dark Half as the crime oeuvre of George Stark, whose recurring hero/alter ego is an evil character named Alexis Machine.
Thad
Beaumont's pseudonym is not so docile as Stephen King's, though, and
George Stark bursts forth into reality. At that point, two stories kick
into gear: a mystery-detective story about the crime spree of George
Stark (or is it Alexis Machine?) and a horror story about Beaumont's
struggle to catch up with his doppelganger and kill him dead.