Time Enough for Love - Robert A. Heinlein

The capstone and crowning achievement of Heinlein's famous Future History, "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.