Short answers, pulled from the story.
De revolutionibus orbium coelestium, meaning On the Revolutions of the Heavenly Spheres, is the seminal work on the heliocentric theory by the astronomer Nicolaus Copernicus. It offered an alternative to Ptolemy's geocentric system, placing the Sun at the center of the universe with the Earth in motion around it.
De revolutionibus orbium coelestium was first printed in 1543 in Nuremberg, in the Holy Roman Empire, by Johannes Petreius. It was published just before Copernicus' death in 1543.
Andreas Osiander, a Lutheran preacher living in Nuremberg, added the unsigned letter Ad lectorem to the front of the first edition. It argued that Copernicus' system was mathematics meant to aid computation rather than a claim of literal truth, a position at variance with Copernicus' own claims.
Copernicus argued the universe comprised eight spheres, with the fixed stars in the motionless outermost sphere and the Sun standing still at the center. The planets revolved about the Sun in the order Mercury, Venus, Earth, Mars, Jupiter, Saturn, while the Moon circled the Earth, and the Earth's daily rotation explained the apparent motion of the heavens.
The Sacred Congregation placed De revolutionibus on the Index of Forbidden Books by a decree of the 5th of March 1616, for teaching the doctrine that the earth moves and the sun is motionless, judged contrary to Holy Scripture. The book was suspended pending corrections and remained on the Index until 1758, when Pope Benedict XIV removed the uncorrected book.
Arthur Koestler called it "The Book That Nobody Read," but Owen Gingerich disproved this through a 35-year study of every surviving copy of the first two editions. He found that nearly all leading mathematicians and astronomers of the time owned and read the book, though most focused on Copernicus' equant-free planetary models rather than the cosmology at the start.