John Carpenter's Starman by Alan Dean Foster

The story follows an alien who crash-lands on Earth and takes on the form of a recently deceased man, much to the shock of the man's widow. As the alien, known as Starman, attempts to understand human emotions and navigate the complexities of human life, he embarks on a journey across the United States with the widow. Pursued by government agents, they form an unexpected bond, exploring themes of love, loss, and the essence of humanity, while racing against time to reach a rendezvous point for the alien's return home.