In 1983, Puzo also wrote The Sicilian, a "sort-of" sequel novel that featured the Corleone family.
The Godfather is still a very valuable brand. Paramount licensed a videogame version in 2006 that included animated portrayals of the characters with the stars' permission and involvement. Original cast members lent their voices to the project, including James Caan, Robert Duvall, Abe Vigoda, and even Marlon Brando before he died. (Al Pacino was not involved because it conflicted with his contractual obligations to a Scarface videogame.)
Paramount authorized a sequel novel, The Godfather Returns, in 2004, but the Puzo estate published another sequel, The Godfather's Revenge, without the studio's involvement or permission. Paramount claims that they hold sole ownership of the Godfather property and are trying to stop the estate from making any further projects based on the material, including a prequel tentatively called The Family Corleone, stating that such "unauthorized" ventures tarnish the legacy and commercial value of the franchise.
In turn, the estate is suing Paramount for $10,000,000, denying the film studio's claim to exclusive rights to The Godfather. The Puzo estate is suggesting that Paramount breached its contract with Mario Puzo (who passed away in 1999) and should no longer have control of publishing rights to the property, enabling Puzo's heirs to profit from Puzo's original characters and story ideas.
I'm reminded of the quote by Michael Corleone to his brother Fredo, "Don't ever take sides with anyone against the family again. Ever." The battle lines are drawn. Will either side make an offer the other can't refuse? Or will this war get uglier before it's resolved?