I always thought it was a trilogy +1.
Volume 1: The Old Testament (Gained God an almost fanatical fan base in Judea)
Volume 2: The New Testament (The long anticipated sequel, which disappointed many fans of the original, but gained God a lot of new readers)
Volume 3: The Last Testament (This came out from a different publisher, who tried to make it look like it wasn't part of the original series by retitling it The Koran. By that time, a lot of the fans of the first two books had lost interest, but it became something of a cult with the younger generation)
Volume 4: The Book of Mormon (This was just fanfic, and not widely distributed)