The BTK serial killer says in a letter from prison that he’s cooperating with a book about the 10 killings he committed from 1974 to 1991 in the Wichita area.

Dennis Rader is serving a life sentence for the killings. His self-coined moniker was BTK, which stood for “bind, torture, kill.”

The Wichita Eagle reports that Rader explained in the letters that he is barred from profiting from his crimes by a court settlement. He signed over his media rights to the families of his victims after he was sent to the state prison in El Dorado in 2005.

The author working with Rader is Katherine Ramsland, who oversees the criminal justice master’s program at DeSales University in Pennsylvania. She says she’s written 54 mostly academic nonfiction books.