Wollstonecraft

A clever, socially deficient young Lady is befriended by a clever girl and form a secret detective agency because it seems interesting. There's also a clever boy, their tutor, and a clever boy who likes to read who make for useful accessories to their activities on occasion.

Wollstonecraft is one of those low-risk children's mystery series. Pleasant enough and tidy, with nearly everything resolved neatly and explained clearly by the end. Just enough loose ends to make a series.