Of Swords and Parasols

Kamen Tantei is a four volume mystery series by Matsuri Akino, the mangaka of Petshop of Horrors.  In it, Haruka and Masato are high school students who write mysteries under the name of Taro Suzuki, but they don't want that to get out.  They're also the sole members of the school's mystery club.  When a classmate Masato has a crush on commits suicide, Haruka suspects foul play and decides to investigate.  During the investigation, a flamboyant masked detective appears and helps them to solve the case, much to Haruka's consternation.

Like PoH, the stories in KT are set up so that each chapter is an individual story, with the ongoing story lurking a bit in the background.  Each mystery is set up to follow certain mystery plot conventions-the locked room, the hotel where the power is out, the mystery writers' convention, etc.-and has many genre scenes and nods to the conventions, such as a scene where Haruka dramatically points and says "the killer..is you!" and a scene in one story where Haruka and Masato are at a dinner where people want to meet "Taro Suzuki," but they can't announce their identity, and the mystery detective arrives and announces that he is Taro Suzuki, and Haruka, though angry, can't do anything about it without revealing the secret.  All of this and more is done with pure, fangirlish glee, and very tongue-in-cheek.  In addition, (sincethis is a Matsuri Akino manga) ghosts have played into every mystery, and "Taro Suzuki" himself seems to be a ghost.  While Masato can see ghosts, Haruka doesn't believe in them.

So far, Haruka and Masato don't seem to be being set up to be love interests.  They're childhood friends who act more like brother and sister than anything else, though with their insistence that they aren't a couple when asked and Haruka's suspicion to girls liking Masato, that could change.  Though they both claim to be mystery fans, I'm not sure Masato actually is.  While Haruka is a complete mystery fangirl(she literally bounces and gets starry-eyed when there's a mystery afoot, or when she gets to meet a writer) Masato, who has a much less dominant personality, mostly seems to be doing whatever she wants out of habit.  "Taro Suzuki" mostly remains a mysterious figure, and has some similarities to Count D.  I suspect he'll eventually be revealed to have some sort of past connection to Haruka or Masato(as he seems more interested in Haruka than Masato at this point, I'm leaning towards it being her.)