He is writing a book for Penguin.

We're told it's a book about Earth and Mars. It will be half about the issues facing us on Earth — sustainability issues in particular. 

The second half will be about the idea of a multiplanetary existence — about what's possible, about the adventure of experience.

Musk's literary agent is Jennifer Rudolph Walsh, who runs an agency called the Worldwide Literary Department.

Walsh invited a group of publishers to hear a pitch on the book. All of them bid. Musk narrowed the field down to two. Penguin flew someone out to meet with Musk at SpaceX's California headquarters. They won the deal.

Multiple sources say Musk will make more than $3 million on the deal. We heard $3.5 million a couple times. One source close to the deal says that's low.