Far and away the best, most enjoyable locked room murder mystery novel I've read. To be fair, I've not read a lot of them, but this one is the easy standout of the bunch. The detective rivalry aspect is really fun, characters don't get backgrounded in favor of mechanical gimmicks, there are smart observations made every other page instead of having a pile of them at the very end and Rouletabille is a delight. Easy recommendation.