In the thriving community of Sylvia Plath and Ted Hughes scholars, the suicide of their 47-year-old son has been deeply felt through the prism of his parents’ writings. His birth was vividly chronicled in Ms. Plath’s journals. He was “the baby in the barn” in “Nick and the Candlestick,” from Ms. Plath’s best-known collection, “Ariel,” written in her final months.