Robert Heinlein's Time Enough for Love is two stories, intertwined. In one story, Lazarus Long, the oldest man in the universe, has grown so old he is tired of life. He wants to die. Seeking death, he is rescued by his descendants and rejuvenated, and rediscovers a reason to live. This story is told in straightforward fashion, from the hero's despair through his convalescence and reconnection with his family. The story does loop back in time: because this is science fiction, time travel is allowed.
The other story is episodic, and takes the form of anecdotes from Long's many lives. These episodes serve to illustrate Robert Heinlein's beliefs about life, love, and the nature of a good society. He seems to have had fun writing them, too.
The first episode concerns a man who was "too lazy to fail". It's a story of an innovator who joins the navy, excels at swordsmanship, gets a pension, invents more efficient tools and methods, and ends prosperous and highly respected, living off the subsidies America pays farmers. Not coincidently, Heinlein graduated from Annapolis, where he was a prize swordsman. While recovering from pulmonary tuberculosis, he dreamed up the waterbed. He never would have accepted a government subsidy though, and his account of his hero accepting one is pure satire.
The second tale, about "twins who weren't", illustrates two of Heinlein's driving social beliefs: everybody ought to stand on their own two feet, and, everybody's sex life is their own business, if it doesn't hurt children. The story includes, in fictional form, directions for making dependent people become independent. It also includes an apology for incest.
To Lazarus Long, incest was only morally wrong if it produced genetically defective offspring. This point of view might be contrasted with that in "Chinatown", for example, where incest is shown to be the act of a monster who wants to own the future, and shown to cause terrible damage to its victims. Heinlein's "twins" also are shown to be equal in power and autonomy, definitely not the situation in most cases of incest. Heinlein makes a reasonably good case for the couple in his story, but theirs is a very special case indeed.
The Tale of the Adopted Daughter is an adventure story, similar to Farmhams Freehold or Glory Road, other Heinlein works. The couple here fight dragons and bandits, and open a new territory to settlement. They build their own farm, breed up sturdy children, and help create a society of like minded rugged individualists. Heinlein admired the pioneer spirit. He actually built his own house in Bonny Doon California, and relaxed as a stonemason between books.
As Lazarus (whose name, of course, refers to the man in the Bible who was raised from the dead) tells these stories to his descendant, he is being rehabilitated. He finds a new interest in life through the creation of two female twins from his genetic material. (He gets cloned, we'd say.) He lights out for the territories of a new planet, and participates in an open marriage there. The differences between Eros and Agape are explored. Time travel is perfected.
Lazarus travels back to his childhood home, and meets and falls in love with his mother. Through love and loyalty, he is swept up in the futility of World War One, and becomes a hero in the trenches of France. He dies, possibly meets God, and is reborn.
Around 1970, Robert Heinlein suffered peritonitis, a life-threatening disease, and it took him two years to recover. At the end of his recuperation, he began working on Time Enough for Love.
Time Enough for Love by Robert A. Heinlein
5830 Bonny Doon Road, Santa Cruz, California (Google map for Heinlein's house)