This is book 5 of the "Flavia de Luce" series.

This note does not contain major spoilers for the book.

English country mysteries, set in the 1950s, and written in the 2000s.

Flavia is 12 in this one I think, but is quite knowledgable about chemistry, especially poisons. And quite comfortable around dead bodies.

Their ancestral house is at stake, not so much because of the mystery exactly. Family history comes up, and there are crazy people locked in attics (or at least luxury apartment suites). There's a great honking diamond, too. And the body of a saint.

I'm always a sucker for precociously-smart children, so that helps.

These were recommended to me by a very small girl (maybe 3?) who pointed out a different one to me (because it had a nice cat on the cover). Looked interesting enough that I dug around to find the earliest one on the shelf, and this was it.