黒白の旅路 (kokubyaku no tabiji, Journey in Black and White, 1977) by NATSUKI Shizuko (夏樹静子) is one of the few Japanese detective stories with an English translation. The translation, by Robert B. Rohmer, was published as Innocent Journey in 1989.
Shinjuu 心中 is Japanese for a suicide pact, normally between two lovers. I don't know how common they actually are in Japan, but they certainly feature more in literature than in Europe. Quite often, too, when you read a writer's biography on Wikipedia, you find a double suicide at the end - or even in the middle: DAZAI Osamu seems to have more or less made a hobby of it (the first time, his partner died and he survived, the second time both he and his wife survived, the last time both he and his partner died).
In Journey in Black and White, the student NOZOE Rikako (野添立夏子) works nights as a hostess in a bar. When her lover, failing company president TOMONAGA Takayuki (朝永敬之), asks her to join him in suicide, she agrees, and the two set off to a mountain region. They lie down together in the forest and take an overdose of sleeping pills; but Rikako does not die. She wakes up in the darkness after vomiting the pills and finds Tomonaga dead by her side, but not of an overdose: he has been stabbed.
This sounds very promising; but the book didn't really live up to its promise for me. The intriguing aspect of the initial situation is quickly forgotten; and neither Rikako (who suffers from depersonalization disorder) nor the reader feels much emotional involvement. Instead the story starts on a "fugitive pursuing the real criminal" story. Rikako runs away, then realises that the police will see her as the obvious suspect. She starts investigating to find the real killer before the body is discovered and the police work their way round to catching her. Her investigations soon lead her to Tomonaga's beautiful widow. Meanwhile a young architect is searching for his missing brother in law; and his search too leads him to Tomonaga's house.
This sounds promising in a different way, the start of a Hitchcock-style hectic action adventure, perhaps. But although there is action enough, it never feels very exciting, perhaps because Rikako is so consistently glum.
The other problem is that the mystery is for the most part very, very obvious. There are two major twists, and in both cases it's a safe bet that readers will be hundreds of pages ahead of the characters on them. In general, this is one of those books where the plot only just about survives because the main characters are consistently picking the stupidest choice of action. The book probably has some interest for attitudes to gender in Japan at that time; in some ways it is a little reminiscent of the kind of books Ruth Rendell and others were writing a decade earlier.