The letters between Sam and Derek continued steadily over the following year, arriving perhaps once a month, careful and measured on both sides — Sam describing school, his growing skill at chess, Bramble’s latest misadventures; Derek responding with careful, uncertain warmth, describing his own slow rebuilding of an ordinary life, a steady job at a hardware store, a small apartment he’d furnished mostly with secondhand furniture and evident pride in having managed even that much stability.

One letter, arriving in early spring, struck Sam enough that he mentioned it unprompted over dinner one evening.

“He wrote something kind of heavy this time,” he said, turning his fork over thoughtfully. “He...