By the Pricking of My Thumbs (1968)

Plot: An old lady goes missing from a retirement home and it's all to do with a mysterious picture.

Well, we're not meeting Tommy and Tuppence in order. Christie's sleuths don't generally tend to change that much, but Tommy and Tuppence are the exception, growing older each time we encounter them. We first meet them just after the First World War and here they're all grown up - although clearly not quite in their 60s.

This is another nostalgia murder, with a clue to a long-forgotten crime and the title being a quotation from a rhyme (well, okay, Macbeth, so it's not quite a nursery rhyme, but it fits interestingly in with other rhyming titles). As Tommy and Tuppence have aged, so has their quarry. As Tuppence puts it, "If you're pretty nasty when you're twenty, and just as nasty when you're forty, and nastier still when you're sixty and a perfect devil by the time you're eighty...", presciently predicting the course of the story before she meets dotty Mrs Lancaster with her question "Was it your poor child?" (a question that apparently will crop up a few times in various books), a red-herring that's vital to the plot of this book.

This is a wild departure from a rigidly plotted Poirot of murder-investigation-revelation. This is more like a teasing quest for something unknown. Is Tuppence looking for inner peace, a missing pensioner or a house in a forgotten painting?

What evolves is a weird first half that should be BORING. Nothing happens. There's a feeling of missed opportunities - Tuppence always turning up too late, or pottering aimlessly around the village where she's staying, always just a few steps away from a mystery. But amid all the small talk and banter, there is a feeling of creeping, creeping menace - of things found in chimneys, mysteries in graveyards, and village gossip no longer repeated. And then BANG! Tuppence goes missing, and it's up to Tommy to rescue his wife.

The second half features corrupt solicitors, a search for clues hidden in a painting, talk of mental homes and a complicated conspiracy being gradually revealed. And it's all rather marvellous. As Tuppence comments when they're re-united "Hearsay, suggestions, legends, gossip. The whole thing is kind of like a bran tub."

But, with careful sifting, the results are suitably rewarding. This is a story that really does pay off. Whereas the nostalgia murder of "Five Little Pigs" is more a clever stylistic exercise, this is a genuine treasure hunt with an obvious prize, a suitably horrible mystery, and everything to reward the reader from secret rooms to fiendish clues.

This is really Tuppence's book. She's a breath of fresh air after the omnipotence and self-confidence of Poirot and Marple. She's just clever, intuitive and genuinely interested in human nature, while at the same time worried that life has passed her by. She gets into terrible scrapes that you wouldn't imagine happening to Marple, and her detecting is methodical, almost plodding, in a way that would have Poirot despairing. And yet... she is immensely lovable because of it.

NEXT: Miss Marple gets A Pocket Full Of Rye