This had a very classic feel to it and reminded me of something like From the Mixed-Up Files of Mrs. Basil E. Frankweiler. The dialogue was very formal and Dev is precocious and precious-- she rhapsodizes about her literary friends Pippi Longstocking, the Artful Dodger and Hercule Poirot. Dev and her friends are all fairly privileged, and are able to do a lot of their investigation because her grandmother is wealthy and well connected.
Readers who adore clue oriented mysteries like Beil's Red Blazer Girls or Runholdt's Kari and Lucas Mysteries will enjoy traveling about with Dev on her investigations and will wait avidly for the next book in the series.