Original: Although it was not Gutenberg's first work, it was his major achievement, and has iconic status in the West as the book that marks the start of the "Gutenberg Revolution" and the age of the printed book.
Simplified: Although it is not the first book to be printed by Gutenberg's new movable type system, it is his major work, and of central importance for the start of the "Gutenberg Revolution" and the "Age of the Printed Book".