Under the Wide and Starry Sky by Nancy Horan
Fanny Osbourne is running away from America with her three children. She's had enough of her husband's cheating ways; surely Antwerp is far enough away. But when her youngest son falls ill and then dies, she's encouraged to recuperate in provincial France. There she meets Robert Louis Stevenson, who immediately falls in love with her. As he's several years her junior, she doesn't initially return his affections. But soon she's under his spell, and thus begins the whirlwind lifetime of land and sea, from frozen mountains to tropical rainforests, in sickness and health, for richer and poorer and until death did them part. This is Nancy Horan's "Under the Wide and Starry Sky."