The Sweetness at the Bottom of the Pie by Alan Bradley is a delightful first mystery. Set at a fading mansion in 1950s England, it features and 11-year-old aspiring chemist (with a profound interest in poisons) as its protagonist. Flavia de Luce is precocious but believable.
This was my second reading - it's this month's Book Club book at the Galesburg Public Library - and I enjoyed it just as much as the first time through. While quite different in style from many "cozy" mysteries, it does not contain graphic violence, sex, or bad language. It's entertaining but refreshingly wholesome (although Flavia and her sisters do play some wicked tricks on each other). There are some plot twists that are not particularly believable, but they do not detract greatly from the story.
The fourth Flavia mystery is due out this fall and I am very much looking forward to it.