In his letter, Harris said his "inner rage" came to the fore after eight months in prison.
"I've started writing a song about the injustice of it all," he wrote, adding that it should have a "country rock sound".
"I plan to record this the moment I get out towards the end of 2017."
He added: "Prison is no hardship really.
"I'm in the art room as an assistant to the tutor and basically I'm doing what I like."