Time Enough for Love is one of Heinlein's later novels, and he uses it to some extent to wrap up some threads from his Future History. The prime character, Lazarus Long, we encountered previously in Methuselah's Children, when he rallied the troops and led the Howard Families into their exodus from Earth to avoid persecution. He is the oldest human being, born at a time when the Howard breeding experiment hadn't really produced any fruit, but long-lived nonetheless.
As the story begins, we find Lazarus in a hospital on Secondus, the seat of government for the Howard Families, undergoing rejuvenation therapy. After a long and adventurous handful of centuries of living, he has grown tired of the whole game, and decided to let himself die naturally of old age, but Ira Weatheral, Chairman Pro Tem of the Families, has had him tracked down and rescued from himself, in a belief that the universe needs the wisdom tucked away in Lazarus' brain. The families have grown stagnant, and Ira intends to con Long into leading another exodus, to break civilization out of its rut.
Lazarus works a Scheherazade-in-reverse deal with Ira, agreeing to proceed to full rejuvenation only if he demonstrates, by hanging around whenever Lazarus requires him, to listen to his tales of wisdom...and woe. So Ira, and/or his deputies and the workers in the clinic, and we gentle readers get to hear Heinlein once more use his favorite bit of gadgetry for preaching - the cranky old curmudgeon who's been everywhere, seen everything, and just doesn't give a damn.
There are several key stories in this book, bound together by the main narrative of Lazarus' healing. The first one is The Man Who Was Too Lazy to Fail, which draws fairly heavily on Heinlein's experiences at the Naval Academy. David Lamb was born and raised on a farm, and decided very quickly that manual labor was not to his taste. Thinking, however, wasn't really like work at all, and he manages to get himself an appointment to a military academy, where he figures he can study all the time, and not do a lick of physical work.
The tricky way he games the system at the academy makes for fun reading. He manages to avoid being forced to play football, which seems too much like work, by rapidly becoming indispensible on the fencing team, and avoids much of the corporal punishment meted out to cadets by a nearly perfect memory and some subtle psychology. After graduation, he becomes a pilot, but not of fighter jets, which could be shot down over enemy territory or the deep blue sea, but of quiet and slow transport aircraft, which don't often get shot down, and even better, allow him to sleep in his own bed with his own wife at night. After his military career ends, he figures out a way to become a farmer again, without the hard work of growing any crops, by taking advantage of government subsidies. Lots to like here.
In The Tale of the Twins Who Weren't, Lazarus, as Captain Aaron Sheffield, gets far more than he bargained for when he buys a pair of slaves and tries to free them. Once again, we revisit the anti-slavery theme he explored in Citizen of the Galaxy. The problem, Sheffield finds, is that when you take responsibility for setting someone free, they may not have the skills to survive as free men, so you also bear the responsibility to teach them how to live as such.
He also explores some related ideas in The Tale of the Adopted Daughter. Sheffield has delivered a shipload of colonists to a backwater planet, and decides to hang around for a while, making sure the colonists have the best shot at surviving. He operates a General Store, where he keeps the value of the "currency" stable by holding prices steady, even if it means personally taking a loss. Some interesting thoughts about economics in this section.
When a fire kills a young couple and leaves their infant daughter behind, he works out an arrangement with a local schoolteacher to care for the girl, while he helps foot the bill. About the time the girl, Dora, becomes a woman, Sheffield has heard the wild goose calling, and is about to leave the planet. Dora convinces him to stay (in the most convincing way a woman can) and he ends up following the call to wander by heading out with her to the most remote, unsettled part of the planet, to homestead there. Some great stuff about survival and civilization in here, similar to Farnham's Freehold.
In the central tale these others punctuate, Lazarus and the people he comes to know and make part of his extended family spend many hours dissecting philosophical and sociological questions, which is entertaining in and of itself. The final tale weaves a bit of time travel into the equation, when Lazarus travels back to visit his own past, when he was a child growing up in Kansas City.
This book is a classic in Heinlein's writings, and in my opinion represents the peak of his writing skills - things began to go downhill far too soon after this.