Alan Bradley's The Sweetness at the Bottom of the Pie is a mystery, with a main character sleuth, Flavia de Luce, who is a 10-year-old girl who's obsessed with chemistry in mid-20th-century England and lives in a decaying mansion--her mother died when she was a baby, her father is so distant he might as well not even be there, and her two sisters' relations to her revolve around pranks and torment.
The setting is primarily a mood evoking backdrop of dusty, mysterious chemistry mechanisms, a decaying manor, a marshy wetness that is somewhat dickensian. This is a character-centered book; Flavia narrates, and we experience the disjointedness of a 10-year-old's thought processes complete with her morbid curiosity at finding a dead man in the family's cucumber patch, her fearlessness, and the simple, absorbing, distracting joy at the freedom of riding a bicycle.
Details of stamp collecting; chemicals, elements, and poisons; and magic pepper this world of academia and all the secret guilt and unshared thoughts and emotions that go along with a stereotypically upper-class British stoicism. But Flavia, being a kid, smashes through all that with endearing, precocious wiles to solve the crime.
This may appeal to nonmystery readers, as the focus is more on Flavia than on the clues themselves. It doesn't feel as though the reader should or could have figured out the whodunnit before the characters.
Looking through what I've read recently, I'd say Flavia shares some appeal with Bod, the young main character from Neil Gaiman's Graveyard Book. That's YA, though, and I'm assuming young protagonists are much more common there. Any plucky young narrators in adult fiction you can think of?