Rep. Gabrielle Giffords and NASA astronaut Mark Kelly. (AP)

The book will chronicle their courtship and the Tucson tragedy when Giffords was shot.

Kelly’s retirement note details his history in the Navy as a pilot flying combat missions in Operation Desert Storm and his time aboard the Endeavour and Discovery space shuttles. He ends with a declaration of optimism in NASA’s future:

I know that as our space program evolves, there are those who will question NASA's future. I am not among them. There isn’t a group more dedicated to its mission or more capable than the outstanding men and women of NASA. Exploration is a critical component of what makes our country great. We will continue to explore and NASA will continue to lead that effort.

Giffords has been recovering well, though doctors have said she still is learning to talk and walk. Though she has moved out of a rehabilitation center, she still needs a 24-hour assistant. Kelly, a Navy captain, has served in that branch of the military and in NASA for 25 years.

The couple is now living in Kelly’s home in League City, a town 26 miles south of Houston.

Read more about Giffords and Kelly here:

Mark Kelly talks about his wife’s fear of being shot

Giffords and Kelly return from weekend-trip to Tucson

My husband, somewhere in orbit