The characters are great - the nosy triplets, the dad who only speaks in long words, the henchman identifiable from some distance by his dove - and the plot is full of twists and jokes and cliffhangers. Every so often we'd work out part of the puzzle just a step ahead of Stuart, making us active participants in the adventure.
An age ago, the great Justin Richards advised me that in constructing a mystery plot, the reader should feel it's all twisty and zig-zag, so they have no idea where it is going; but at the end, when they look back on the route they have come, it should be a dead-straight line. I could see exactly what he meant when I read Dashiell Hammett, and here it is for kids. Exciting, funny, rich - and immensely satisfying.