Complex sentence: One of Mullen's last projects, which he was unable to complete before his death, was helping the Red Hot Chili Peppers co-author their biography, which was released on October 19,2009, a little over a year after Mullen's death.
Split into simpler sentences: One of Mullen's last projects, which he was unable to complete before his death, was helping the Red Hot Chili Peppers co-author their biography. The book was released on October 19,2009, a little over a year after Mullen's death.