I'm always glad when the Nobel Prize winner turns out to be an author I've actually read (and this happens less often than I like to admit). I've only read one Doris Lessing novel, 1989's The Fifth Child, but the book has stuck with me all these years.
The Fifth Child is a fable about a happy family. They have one child and everything is great. They have another and everything is great. Then another, and another. Now they have four wonderful children, but as they prepare to welcome a fifth several members of the family begin to suffer from unexpected feelings of dread. Indeed, the new baby arrives looking strangely primitive, almost monstrous, and he doesn't seem to be tuned in to the same sense of joy and togetherness that the rest of the family thrives on.
The story veers towards the disturbing and the tragic, and Lessing's message seems clear: there is an invisible line between blessed happiness and self-indulgent over-happiness, and this line is all too easy to cross. There's also the slightest suggestion that the fifth child is not actually intrinsically different from the rest, but rather that the perfect family found itself unable to extend its love to yet another newcomer.
I remember liking this short novel's dissembling ending, in which the creepy fifth child turns out not to be notably evil (as the family feared) but rather seems to fit right in with the larger British population. After all the fear and dread he inspires in his own home, the fifth child gradually leaves to join the massing rejects that make up the outside world, and ends up simply lost in the crowd.
I'm describing all of this from memory (and I hope I have the plot points right). The fact that the plot has stuck with me since 1989 must mean something. Congratulations to Doris Lessing, our latest Nobel Prize Laureate.