Time for the Stars

Robert A. Heinlein

This is a coming of age story about a set of twins who embark on a journey of exploration in space, but one never leaves planet earth. One grows old, the other ages much more slowly due to the relativity aspects of the journey. The explorations are more than just in space and time; they also deal with the exploration of self. Like most of Heinlein's novels, this story has a delightful optimism.