I’m currently reading 首切りサイクル by Nisio Isin and really enjoying it. It’s on the longer side and it was fairly difficult at first, but after the first hundred pages or so it’s been such a page turner that I can’t put it down. After getting used to the vocabulary and the distinctive way in which the narrator and some characters talk it’s gone fairly smoothly. It’s a locked-room mystery with Agatha Christie vibes.
If it seems to be a bit much, Nisio Isin has many other well-regarded mystery novels that are a bit more digestible. I definitely intend to read more sometime down the line.
Also, the upcoming Intermediate Bookclub Pick 地球星人 is not a mystery novel per se, but it is definitely an edge-of-the-seat page turner that uses relatively straightforward language, and you’d have the support of a bookclub.