This is a very early Christie. It is not, strictly speaking, a mystery, but rather an action adventure that might have made a good serial in a Saturday morning matinee. It's a bit silly and far fetched, but never dull.
This book brings back several of the characters from "The Secret of Chimneys". The story gets a bit bogged down with several possible motives branching in many different directions. A worthy denoument makes this book.