On June 24, 2015, Dawn Dorland, an essayist and aspiring novelist, did perhaps the kindest, most consequential thing she might ever do in her life. She donated one of her kidneys — and elected to do it in a slightly unusual and particularly altruistic way. As a so-called nondirected donation, her kidney was not meant for anyone in particular, but for a recipient who may otherwise have no other living donor. Several weeks before the surgery, Ms. Dorland decided to share her truth with others. She started a private Facebook group, inviting family and friends, including some fellow writers from GrubStreet, the Boston writing center where she had spent many years learning her craft. After her surgery, she posted something to her group: a heartfelt letter she’d written to the final recipient of the surgical chain, whoever they may be. Ms. Dorland noticed some people she’d invited into the group hadn’t seemed to react to any of her posts. On July 20, she wrote an email to one of them: ...