It's fairly rare for Rex Stout to have a fully "classical" detective story, with a relatively limited cast and a "deductive" approach to determining who did what, or at least a fair challenge-to-the-reader.  This might be the best of them, when Wolfe is presented with an impossible task that Stout seems to delight in making more and more impossible with every twist and turn; yet, to Wolfe, each additional complication also is revelatory, until he knows everything, and if you closely look back in the book, you can too.  It's great watching Wolfe get skunked and then slowly turn it around. You do truly feel like Wolfe is a genius at the end of this even though he berates himself for his stupidity.