Kimi to Boku ga Kowashita Sekai

Recap: In Kimi to Boku no Kowareta Sekai, Hitsuuchi Samatoki's romance with his sister Yoruichi is interrupted by MURDER. The case is eventually solved with the help of Byoinzaka Kuroneko, who attends school in the nurse's office because she is pathologically afraid of people. She also sleeps with anybody who asks her to, a fact Nisio appears to have conveniently forgotten in the five years between these two books.
In Bukimi to Soboku no Kakomareta Sekai, Kushinaka Choshi's sister is murdered, and he sets out to solve the mystery with the help of Byoinzaka Meiro, Kuroneko's cousin. Meiro does not talk, but communicates volumes of exposition through facial expressions alone. Kuroneko herself makes an appearance near the end, to tie up some loose ends and make sure we all understand that Choshi is the creepiest fuck Nisio's ever created.
The third novel finds Kuroneko (narrating) and Samatoki on an airplane. Kuroneko's distant relative, Usui, has funded a trip to London to help out an old friend of his -- a mystery novelist who believes his new novel is cursed, and anyone who reads it will die. Their flight is interrupted when the man sitting next to Samatoki is murdered.
Chapter two kicks off with Samatoki (now narrating) reading the short story Kuroneko wrote on the plane, in which the man next to them is murdered. They meet with the mystery novelist, and then make a beeline for 221B Baker Street on the grounds that this is clearly the first place anyone in their right mind would visit in London. (Samatoki would have preferred to see the Rosetta Stone.) Along the way they solve the mystery of how the mystery novelist faked his wife's suicide.
Chapter three kicks off with Kuroneko narrating again, and pointing out that the previous two chapters were actually written by Kushinaka Choshi, who actually accompanied her on this trip. And of course, the mystery novelist's wife actually died in a plane crash. As they tour the British Museum -- or rather, Kuroneko does, since Choshi refuses to stop staring at the Rosetta Stone -- they solve the mystery of the mystery novelist's agent's suicide.
Chapter four kicks off with Samatoki narrating, and demanding to know just who the hell Choshi is and why Kuroneko replaced him. Of course, the agent actually died of a heart attack, and the only thing they've been sent here to do is prove the novel isn't cursed, which is why Kuroneko just finished reading it. In the morning, Samatoki finds her dead, with an army knife through her chest. He goes to see Phantom of the Opera, but fails to solve her murder.
Chapter five kicks off her absolutely furious with him for killing her off in his chapter of the novel they're writing, particularly since he failed to actually come up with a solution to the mystery. She didn't read the novel, but instead pointed out that it couldn't be cursed if the novelist himself was still alive. At this point Usui calls their room and tells them the novelist has been found dead...and the time of death is the day before they actually met him.
The epilogue has them both at the airport ready to go to London. Usui -- who is actually Kuroneko's father -- paid for the trip in return for their feedback on the novel he'd written. The trip is then canceled due to terrorism in London.
Gloriously meta from beginning to end. I'm clearly going to have to go get the fourth book in the series now.