Time Enough for Love - Robert A. Heinlein

The capstone and crowning achievement of the Future History series, from the New York Times bestselling Grand Master of Science Fiction... Time Enough for Love follows Lazarus Long through a vast and magnificent timescape of centuries and worlds. Heinlein's longest and most ambitious work, it is the story of a man so in love with Life that he refused to stop living it; and so in love with Time that he became his own ancestor.