The Fifth Child by Doris Lessing.
David and Harriet Lovatt aren't one of your groovy modern couples. They're a bit staid, a bit old-fashioned, you might even say, but after meeting at an office party and recognising each other as kindred spirits they're very much in love and have set up a nice little life for themselves in a sprawling old house, with plenty of room for the unfashionably big family they're planning to have.
Sure enough Harriet soon starts firing out offspring like bullets: Luke, Helen, Jane and Paul. Money is a bit of an issue, but David's father has a bit of spare cash knocking about between yacht purchases so he's prepared to help out, though the general consensus seems to be that David and Harriet should slow down a bit and perhaps discover the joys of effective contraception.
No such luck, however, as Harriet soon finds herself pregnant once again. This one seems different, though - painful and troublesome in the womb, and things don't improve when he finally batters his way out a month early. Ben is a strange, troll-like, Neanderthal creature quite unlike the angelic children that preceded him. But what is he really? A changeling? A throwback? It seems it's a bit more than just his generally unprepossessing appearance, as he lacks the other children's cheery disposition as well, is slow to learn and seems to have a generally malevolent streak. The other children soon learn to steer clear of him, and possibly with good reason as a couple of pets meet mysterious deaths and suspicion falls upon Ben.
Eventually Harriet and David find their home life disrupted to such an extent that they have Ben committed to an institution. The other children are delighted by this and home life returns to something approaching normality, but Harriet is racked with guilt and eventually makes the long drive north to retrieve Ben from the hellish existence he has been consigned to. Which is all very commendable, of course, but means that he's now back at home, with a good dose of post-institution trauma to add to his existing problems.
With Ben back the large family Christmas gatherings peter out, and as soon as the older children can they drift away from the house as often as they can, and eventually permanently. Ben, now approaching his teens, drifts into hanging out with a local motorcycle gang and, once he reaches secondary school, acquires a gang of followers of similarly misfit looks and behaviour. The book ends with Harriet and David seeing Ben on the TV news on the sidelines of a city centre riot, and considering selling the house and moving away (and, we're invited to infer, not telling Ben where they're going).
From my pitifully small sample of Doris Lessing's output I'd say the pattern that emerges is that the earlier ones like Briefing For A Descent Into Hell (1971) and Memoirs Of A Survivor (1974) are the weird hallucinatory ones, and the later ones like The Good Terrorist (1985) and The Fifth Child (1988) are told in a much more straightforward and linear way. I think the key to understanding The Fifth Child is to realise that the whole story is essentially told through the viewpoint of Harriet, who may or may not be completely reliable. Are we to assume that Ben is really as he's described, or is it just that Harriet can't cope with someone who diverges even slightly from her idealised notion of what a perfect child should be like (i.e. like her previous four angelic children)? Is Ben really the hideous troll-like creature of Harriet's description, or maybe just a bit slow, or maybe on the autism spectrum somewhere? So you can view it as a straight horror story of a family being menaced by a goblin-like changeling, or you can view it as a satire of nice middle-class liberal horror at having to cope with a child with mild mental impairment, who clearly isn't going to win any Olympic swimming medals or the Nobel Prize for Physics or anything like that, and their inability to come to terms with that being projected onto the (blameless) child. Clearly there are echoes of other books here, most notably We Need To Talk About Kevin, though as here I should point out that (apart from the suspicion over the pets) Ben hasn't actually killed anyone yet.
Speaking of Nobel Prizes, Doris Lessing became the oldest ever winner of the Nobel Prize for Literature when she was awarded it in 2007. As befits a sparky 88-year-old she gave no concession to the various reporters who wanted a soundbite out of her after the announcement was made. And while I think Briefing For A Descent Into Hell remains the one to read, The Fifth Child is (at only 159 pages, and large-ish print) a disturbing and thought-provoking little book and well worth a look.