Forty-one years after a stranger broke into his penthouse with a stolen photograph in his hands, Lucien Ashford sat on the same terrace where he’d once married that same stranger, now considerably grayer and slower than either of them liked to admit, watching a garden Ezra had spent four decades slowly, stubbornly perfecting despite an early complete absence of any discernible gardening talent.

The notebook beside him had grown, over the years, into an entire shelf of volumes — more than two dozen now, spanning from a frightened, uncertain first entry written by a man who hadn’t yet trusted his own capacity for happiness, through decades of ordinary joys and...