After I finished the wonderful The Stolen Child, I started reading I am Legend (1954) by Richard Matheson. Funnily enough, both books are about what it means to be human, but in radically different ways.
The protagonist of I am Legend is the only human being not to have been infected by a plague of vampirism that has swept over the whole earth. As a consequence, he decides that he must kill as many vampires as possible, and hopes eventually to find some other uninfected people. Gradually, however, he works out the infection vector and lifecycle of the vampiris bacteria, and how it accounts for the symptoms of vampirism. It gradually dawns on the reader that if this is an illness, it is not supernatural, so there is no "moral obligation" to kill the vampires. Eventually you get to find out how the vampires feel about it. It's a study in how someone can turn into a complete psychopath, but feel entirely justified in his compulsion to kill. Although it's about vampires, it can actually be classified as "hard SF", since there is a well-worked out scientific explanation of vampirism, and it's about the sociological and psychological developments that might occur if there was a plague of vampirism. But it is also a story of loss and grief and fear, and the possibility of forgiveness. The character of Ruth is particularly interesting, though not that well-fleshed-out, and I'm not sure I believed in her forgiveness. But still, it was a very interesting read. There was also the idea of what it is that makes us human - is it that we are not vampires or whatever, or is it that we are capable of relating to others, of empathy, and trust and acceptance? This aspect of the book sort of reminded me of Cassandra in Doctor Who, who is also "the last human" but in her solitude and pride, has lost her humanity. This loss of empathy is applicable to many other bigots who demonise other people.