Man, these Sugimoto Ichibun covers remain gorgeous. And creepy.
Kamen Butoukai is one of the later Kindaichi novels by Yokomizo Seishi, only followed by Byouinzaka no Kubikukuri no Ie and Akuryoutou. Its birth was also quite troublesome. It had originally been planned and announced as a direct-to-novel release with the slightly different title Kasou Butoukai, but it was never actually released. Then it became a serialized novel in Houseki starting in 1962, but the following year, Yokomizo had to pause the serialization due to health problems. By then however, the Japanese mystery scene was already focusing more on the social school mysteries like those of Matsumoto Seichou, and Yokomizo seemingly lost interest in writing the Kindaichi Kousuke novels, which were basically the anti-thesis to the social school novels, being gothic, fantastical mysteries with a lot of theatrics and drama. But after the success of the manga adaptation of Yatsu Haka Mura, the re-releases of his Kindaichi novels as mass market paperbacks which ignited a genuine "Yokomizo Boom", he got interested again, and in 1974, Kamen Butoukai was finally finished via a direct-to-novel release. Twelve years is quite a long time to finish a book!
But as the book was started in 1962 and the social school boom had already been on-going, you can definitely feel some of that must have influenced Yokomizo, and this book (like the previous novel Shiro to Kuro) feel surprisingly "modern" compared to the more famous Kindaichi novels, which are all set soon after World War II, instead of 15 years later. While the war does still play a role in the book (the Fuenokouji clan is former nobility, the war had affected not only Chiyoko's career but also her bond with Misa, as Misa and her grandmother had to evacuate Tokyo during the bombings, meaning they lived apart for a long time), the world does feel less weighed down directly by the war, and while this story doesn't take place in the city, Karuizawa is still a very popular resort town for more rich people, again quite different from the isolated mountain village or island you'd be familiar with from the more famous Kindaichi novels. Especially for those who have read many of those, this book will feel strangely refreshing.
That said, the book does still follow the usual tropes of a Kindaichi novel, being very focused on digging into complicated family relationships with hidden histories, and these relations being strongly connected to the motive behind the deaths. Very "obvious" tropes like the dying message are used far less often usually. The meaning behind the matches is actually pretty ingenious, but completely impossible to guess before Kindaichi explains what it means. Even if you know what the clue is actually indicating, you can't possibly ever tie that piece of information to the matches, even if in hindsight, it makes sense. In fact, the piece of information it refers to, is something I have seen in quite a few mystery stories, and I think it is used pretty clever here. There is another clue indicating the same thing in this book that on a creative level has much better potential, but it used in a very weird manner, basically showing you the clue, and immediately telling you what it is, which takes away so much of the surprise, as it would have been much better if we had been first shown this scene, and only later been told what it actually meant. Now you get a very vital clue about 70% in the book, while the solving doesn't actually occur until the 90% point. The thing it indicates is used cleverly though: while it used in conjunction with something else that seems a bit obvious and tropey, these two elements work together well to create some misdirection, and I do like it as a clue on its own. It just could have been presented to the reader in a somewhat different manner to make the revelation feel a bit more surprising, and also earned.
Yokomizo does a good job at weaving a complex web of people moving about in Karuizawa around the time of the Maki death and how people's actions will influence other people's actions, though some parts feel a bit odd. The book actually opens with two people committing a love suicide together, but Kindaichi stumbles upon them and calls for help, though he sadly only managed to save one of them. The way this prologue becomes connected to the deaths of Chiyoko's exes is quite forced, and some might even think it feels cheap, as it basically forces one character to behave in a certain way because... the book needed them to do that, but also give them some kind of motivation for doing so, but it doesn't really work.
There are some nice Christie-esque twists in the plot, that worked really well here. In a way, the book feels very much like a Kindaichi novel, but at the same time, it also subverses a lot of the tropes, like via the more modern setting as mentioned, but also the role of Chiyoko in the book. Even the final confrontation with Kindaichi will feel somewhat familiar, though I think this is one of the better times in the series, as the actions of this culprit were really horrible and created a huge tragedy, but in a very different way than in other Kindaichi novels, while still very rooted in reasons that, in a roundabout way, seem to make sense from their point of view. I don't think this one is an absolutely must read (Yokomizo himself did rank it no. 7 out of his personal top 10 Kindaichi novels he wrote), but it is quite fun to read especially if you are already familiar with the better-known books (that follow the classic tropes).
So I wouldn't recommend Kamen Butoukai as anyone's first meeting with Kindaichi, but if you have read a few already, you might find this book very refreshing, set in a very different time period like Shiro to Kuro, and with some elements that feel almost subversive for the series. Not A-tier material, but a solid B. Despite me saying it feels subversive at times, this is still however very clearly a Kindaichi Kousuke novel, and there's plenty to recognize here (the complex human relationships!) and if you like that part of the series, you'll be satisfied with this book too.