Complex sentence: Baronius published a new edition of the Little Office of the Blessed Virgin Mary, complete with all Gregorian chants in October 2007, being the first book to contain the complete music for the office.
Split into simpler sentences: Baronius published a new edition of the Little Office of the Blessed Virgin Mary, containing all of the relevant Gregorian chants in October 2007. It was the first book to contain the complete music for the office.