Complex sentence: Henry Archer (died c.1642), also known as John Archer, was a Fifth Monarchist whose book "The Personall Reigne of Christ Upon Earth" (1642) was popular in the 1650s.
Split into simpler sentences: Henry Archer (died c.1642), also known as John Archer, was a Fifth Monarchist. His book "The Personall Reigne of Christ Upon Earth" (1642) was popular in the 1650s.