I don't think I have many books with a grey cover...
Maya Yutaka is not an author you're going to read if you want something conventional. Ever since he made his debut in 1991 with Tsubasa Aru Yami - Mercator Ayu no Saigo no Jiken ("Darkness with Wings - The Last Case of Mercator Ayu"), you can detect a theme in his works, and that is deconstruction. Maya obviously loves the mystery genre, but it's his love that also allows him to deconstruct the familiar tropes and themes of the genre. Post-modernist themes like the Late Queen Problem are subjects that play a big role in Maya's work, and notions like the truth or the detective, which seem like very "obvious" themes in the mystery genre, are transformed, transfigured and molten into new shapes in Maya's books. His books are seldom straightforward, and will turn the conventions of mystery fiction around just to mess with you, whether it's by not providing a clear truth at all (like in the infamous Natsu to Fuyu no Sonata) or just delving into Late Queen themes like multiple truths/interpretations in Sekigan no Shoujo.
And in a way, it's incredibly fun. But you definitely need to be in the right mindset for this.
It's also very difficult to write about these stories, precisely because more often than not, they're not really meant to be normal mystery stories. They almost feel like punchlines, building on Late Queen problems and other post-modernist themes regarding mystery fiction and taking their conclusions to the extreme. So while the stories start out in familiar ways, the part that is usually the "solution" to a mystery is changed in these stories, concluding in very surprising and subversive ways.
The opening story Shisha wo Okosu ("To Wake the Dead") for example starts very conventionally: Mercator is hired to investigate the death of someone, who died one year ago, while he was staying with some friends in an old house up for sale just outside the town. They had been drinking, and he had been resting in another room, and it appears he just fell out of the window, killing himself. Mercator is now to see whether it was really an accident, as the friends still feel something was off. While they are waiting for Mercator and Minagi (a mystery author who acts as Mercator's Watson/slave), they quickly realize the death wasn't quite normal, and they start to suspect each other, but then Mercator arrives and he... solves the case? I do really like some of the deductions Mercator presents in this story. They do remind you that Maya was indeed a member of the Kyoto University Mystery Club, and he utilizes some clever Queen-like deduction chains to slowly point towards a surprising truth behind the death one year ago. Only... that's not all. Once the dust has settled, Mercator basically turns everything around in a way that completely renders this a non-mystery story in essence. I like the idea, but this first story proves right away this isn't really a mystery collection.
The second story Kyuushuu Ryokou ("A Trip to Kyuushuu") does something similar: Mercator forces Minagi to open a file on his own computer, which turns out to contain a virus. Because Minagi's manuscript has been wiped, Mercator offers to present Minagi a plot for a new story. They walk out of Minagi's apartment, but just near the stairs, Mercator notices a weird smell from one of the neighboring rooms, and when he peeks inside, it turns out someone's dead inside. Mercator and Minagi look around the room, trying to figure out why Minagi's neighbor is dead, as this will serve as Minagi's new story. Again, this story features a few nice short deduction chains based on the physical evidence they find, but ultimately, it's all building up to a climax that can only be described as a punchline, and any feeling of catharsis of learning the solution is washed away immediately. It's quite funny and I do think this particular story is the closest to a "normal" detective story, but still very subversive.
The third and fourth story Shuusoku ("Convergence") and Kotae no Nai Ehon ("A Picture Book Without Answers") can't even be explained properly, as both are truly something you have to experience yourself. In Shuusoku, Mercator and Minagi are invited to a small island with a mini-cult to investigate a break-in, while in Kotae no Nai Ehon, the murder on a teacher is the subject of an investigation. While again both investigations do feature clever deductions that would've been great in straightforward detectives, Maya then decides near the end to tie explosives to the story, blow it up, gather the pieces and then set fire to them just to finish them off. The conclusions of both stories will infuriate you if you want a normal detective story, but that's what makes them so interesting as experiments regarding the set rules and tropes of mystery fiction. Kotae no Nai Ehon in particular is daring with what it tries to do, and truly something you can only pull off once, though I like the surprising elegance of what is done in Shuusoku better. These two are the must-reads of the collection, just for their craziness.
The final story Misshitsusou ("The Locked Room House") is a short short, involving just Mercator, Minagi and a dead body they find one morning in the house they are staying at. What follows is a short conversation-only story that once again ends with a ridiculous resolution if you'd think about it from a "mystery genre" point of view, though at this point, I think a lot of readers can guess what Mercator will do considering how outrageous he's been behaving all this time. The title of the collection is Also Sprach Mercator, and that is certainly a theme running through all these stories: ultimately, it's Mercator who decides what the "truth" is and how each story will end.
I did enjoy Mercator Kaku Katariki a lot, but it's not something I would want to be reading all the time. It worked for me, precisely because I had been reading a lot of formal mystery stories at the time, and so I had a lot of fun seeing those same tropes played with in such a daring way. I do think this collection is the most accessible work written by Maya where he addresses these deconstructive themes. Something like Natsu to Fuyu no Sonata just requires a lot more dedication from the reader, while Mercator Kaku Katariki is actually a very short and light-weight read. This is not a book I can unconditionally recommend to mystery fans, but if the idea of playful deconstruction appeals to you, I think this will be a very fun read.