I've looked at several books by OKAJIMA Futari now, and each one has been quite different from the others. 解決まではあと６人 5W1H殺人事件 (Kaiketsu made ha ato rokunin: 5W1H satsujinjiken, Six People to go to the Solution: the 5 W 1 H murder case, 1985) is a detective story; but it's in no way a fair play mystery, and it doesn't have one detective, it has six, each solving part of the case. The names of the chapters are Who, Where, Why, How, When and What, the '5 W 1 H' of the subtitle. Each begins with a different private detective being given a problem to investigate by a young woman calling herself HIRABAYASHI Takako (平林貴子). Japanese private detectives are a pretty competent lot, apparently; and each of them finds the answer sought, while unaware of the larger mystery that links the various problems.
Now this is a really interesting idea for a book; but in the end the execution doesn't quite work for me. The different detectives and their different problems are interesting to start with. There's the contrast of characters from detective to detective, and there's variety in the different mysteries they have to solve, finding the owner of a camera, finding a café with green matchbooks and a name beginning with two Vs, code breaking, etc. And as the story progresses, the reader can puzzle over how the different problems fit together. From the second chapter on, we know more than each investigator, as they come across names and places about which we already know. In this way the story gets gradually more interesting as it progresses; but the integration of the various parts of the mystery was a little disappointing. The urgency of the story increases as we get closer to the end, especially with the introduction of a less ethical pair of detectives in the fifth chapter. But to get to the point where we can see the outline of the case requires a dump of previously unavailable information at the beginning of the sixth chapter. From there the remaining mystery plays out like a fair play puzzle; but there isn't much room left for surprises by this point.
That probably sounds pretty negative; if it does, I've given the wrong impression. I didn't hate it and I wasn't bored; I'm just disappointed that a promising idea didn't amount to a better book.