From the Mixed-Up Files of Mrs. Basil E. Frankweiler

Yay! This was such a treat. What kid (or adult) hasn't fantasized about running away from home and having an adventure? Claudia and Jamie Kincaid, the runaways in question, leap out of the book so realistically as children who often bicker and are often thoughtless, yet are wide-eyed with curiosity, adventure, and appreciation for life. I saw quite a lot of myself in Claudia, in fact; we both love a good adventure, but even moreso do we love comfort and fastidiousness than dangerous situations.
The last ~40 pages earned the book a fourth star–easily the best part of the book; it's bittersweet and a nice wrap-up for the book's overarching mystery.
The narration/POV could potentially confuse younger readers; it is not from Claudia or Jamie's perspective, but from Mrs. Basil E. Frankweiler, who does not enter the story as a character until well into the book. Then again, having her narrate evokes a sense of mystery and the clues falling into place as the children learn more about her. She begins the story as a faceless, rather curmudgeonly old lady, but at the end of the story she is solved as much as the story's bigger mystery is. She's also a great foil for Claudia's whimsical nature.
Overall, this is a cozy mystery well-suited for kids and adults alike. Really enjoyed it!