Endless Night by Agatha Christie.
Mike Rogers is a little bit WHOOOAAA, a little bit WHEEEYYY; he is, in short, a geezer. Well, perhaps that's a little bit harsh, but he is very keen to transcend his humble origins and has done a series of jobs enabling him to get a sniff of, and a hankering for, the lifestyles of the rich and famous. While working as a chauffeur he has made the acquaintance of one Rudolf Santonix, noted architect, and so when wandering aimlessly in the countryside one day and spotting a crumbling property in some extensive grounds he spends some time imagining in his mind's eye the property replaced with one of his own design, made real by Santonix and his associates.
After a few conversations with people in the village it transpires that the house and its grounds are for sale, and moreover that the property is known locally as Gipsy's Acre and is rumoured to be cursed. But Mike buying anything like this, even at a relatively knock-down price at a public auction, is just a crazy fantasy. Almost immediately he meets American girl Ellie Goodman, also mooning around the property's perimeter gazing wistfully at it, and they strike up a tentative romance. It transpires that she is actually Ellie Guteman, heiress to the Guteman millions and on her imminent 21st birthday set to become one of the richest women in the world.
Ellie returns home for her birthday and Mike attempts some sort of reconciliation with his mother, who seems oddly cold towards him. On Ellie's return, Mike tells her that the Gipsy's Acre property has been sold, and she's like no shit, Sherlock, I bought it for us to build a house on and live in and I've hired your mate Santonix to do the job. Now marry me, you crazy fool.
They marry in a quiet ceremony, and as Santonix and his team get on with building their dream home Mike has to come to terms with the small army of hangers-on associated with someone fabulously wealthy: relatives, ex-relatives, lawyers and Ellie's personal assistant and general companion Greta Andersen, whose loyalty to Ellie regarding the secretive nature of the marriage leads to her getting fired by the family and taken on on a personal basis by Ellie. This results in Greta coming to live with them, an arrangement that starts as temporary but ends up semi-permanent, much to Mike's chagrin.
Mike and Ellie integrate themselves into village life and meet some of the locals - Ellie is a keen horsewoman, which helps, despite having to take pills to overcome an allergy to horses. Mike is down at an auction with one of the locals, Major Phillpot, one day, expecting to be joined by Ellie at the pub for lunch, and when she doesn't turn up he and the major (after finishing their lunch first, obviously) head up to the house to look for her. No sign of her at the house, but when they search the grounds they find her on one of the riding trails, on the ground as if having fallen from a horse, no obvious external signs of injury but clearly dead.
A full-scale investigation is mobilised, and suspicion falls on the mysterious red-caped figure seen near the riding trail by one of the locals during the time Ellie was out riding. Could it have been the old gipsy woman from earlier? Or someone else? Meanwhile the full machinery of the legal apparatus associated with administering the Guteman millions grinds into action and Mike, as sole beneficiary of Ellie's will, is subject to much attention and advice, not all of it well-intentioned. He consults with Ellie's American lawyer, Andrew Lippincott, who undertakes to ensure that Mike isn't swindled but seems cold and unfriendly and tells Mike that he has sent him a letter which he will receive when he returns home.
Mike has some time to reflect on all this as he returns home by sea, the ship having to navigate around some MASSIVE PLOT SPOILERS on the way. Mike reflects that this business has all been rather trying but at least he's returning home to be with the woman he loves. WAIT A MINUTE, you'll be thinking, didn't she die a couple of chapters back? Well, no, because the woman Mike is referring to here is (dramatic orchestral stab) none other than Greta Andersen, who is, it turns out, the real love of Mike's life and the co-hatcher of a fiendish plan to ensnare an heiress, bump her off and then have it away with all her lovely money.
So, yes, Mike, who we'd thought was nothing more sinister than a bit of a directionless chancer made good by marriage to a sweet and obliging woman (who also happened to be, y'know, a millionairess), turns out to be the murderer, managing to be somewhere else when the murder occurred by poisoning Ellie's horse allergy pills, knowing she was intending to ride that day. It also transpires this isn't even the first time he's killed someone, having offed a childhood friend by pushing them under some ice (stealing his watch into the bargain, the swine) and then finished off an army colleague, seemingly just for psychopathic shits and giggles, after he'd been stabbed.
Mike returns to Gipsy's Acre and Greta but imagines he sees Ellie as he approaches the house, and finds Greta less than receptive to the idea of staying on and making the place their home - she favours selling up and scarpering, which seems, on balance, a better idea. Mike's precarious mental stability is further undermined when he finds and opens the letter from Lippincott and discovers a newspaper cutting featuring a photograph of him and Greta in Hamburg dated well before he and Ellie ever met. So Lippincott knew!? Or knew something dodgy was afoot, anyway. Greta shouts at him to pull himself together but he responds by strangling her, at which point Major Phillpot and the police arrive and the jig is up.
I read what must have been about twenty Agatha Christie novels in what must have been my late teens and early twenties after we inherited a large stash of them from my grandmother. This one is one of the last novels she wrote, in 1967 (she died in 1976 at the age of 85) - the "classic" Christie period is the subject of some dispute but is generally regarded as running from the early 1920s when she started writing murder mysteries to perhaps the very early 1950s. It's also unusual in the Christie oeuvre for not featuring any of her recurring detectives, most notably Hercule Poirot and Miss Marple, and for having all the murdering and revelations happening very late in the book rather than right at the start - Ellie's death is revealed on page 139 of a 191-page book and Mike only explicitly outs himself as the villain on page 173. The device of having the book's narrator also be revealed as the murderer isn't completely new, even for Christie - it's more famously used in The Murder Of Roger Ackroyd in 1926, a Christie I have, as it happens, never read.
Endless Night is generally regarded as the best of the late Christies and it is suggested it was one of her own personal favourites. And it is very good, really more of a twisted romance slash psychological thriller than a murder mystery in the orthodox sense. Nonetheless as with all mysteries there are some implausibilities - Mike seems to have no trouble getting hold of some cyanide when he needs it (and no-one seems fussed about toxicology tests and the like after Ellie turns up dead), it's unclear how he and Greta engineered his meeting with Ellie, the whole business with the newspaper cutting seems a bit unlikely, and Mike's mental disintegration on returning to Gipsy's Acre seems a bit sudden, given the cold-blooded bastardry of the plot he's just engineered - but as always this isn't really the point, and some of it can be handwaved away with a weeeeell, unreliable narrator, whaddaya gonna do? There is also just a suspicion in the middle section describing Mike and Ellie's life after getting married and settling into life in Gipsy's Acre of Christie tapping her watch and going: have I written enough pages to get to the murdering bit yet? Perhaps I'd better throw in another vaguely sinister and possibly curse-related occurrence to keep the tension up.
Anyway, despite its unorthodoxy as a murder mystery Endless Night has been adapted for the screen a couple of times - the TV adaptation contrived to shoehorn Miss Marple into the mix somehow, while the 1972 film kept a bit more closely to the book's plot.