Susanna Clarke's Piranesi is a novel in the form of a series of journal entries; the character who is writing is, by turns, unreliable, forgetful, and unclear. But because the novel would truly suffer from continuous vague recollections, he also has perfect memory for dialog and for visual descriptions of scenery and surroundings. Does this seem contradictory? Yes, but lucky for us, it doesn't matter because nothing in this book particularly matters.
The novel takes place inside a giant House --- a truly enormous House --- a possibly endless House, consisting of a series of marble-lined, colonnaded halls, vestibules, staircases, and passages. While the narrator uses the word "House", readers and other incidental characters come to understand the series of rooms as a labyrinth: not connected in any predictable way, challenging to navigate, and full of distracting detail. There is no obvious entry or exit point, though the action is centered around a vestibule (also the origin point whence the narrator indexes all other rooms). The narrator is vague on details that seem significant (how does he remember navigation directions perfectly when others get so easily lost?) and incredibly specific on details that no one else cares about (how many daily-use goods will he fashion from "fish leather", where is the best room to go bird-watching from, what is the Platonic Ideal Good Action for him to take in any situation). He confusingly both denies the existence of a world outside the building AND knows lots of words that refer to things that exist in our world, but not in the House.
This sounds mysterious, and it is. Clarke is a good writer and this book has many excellent moments --- e.g. the wink to the reader on p. 60,
I realised that the search for the Knowledge had encouraged us to think of the House as if it were a sort of riddle to be unravelled, a text to be interpreted, and that if we ever discover the Knowledge, then it will be as if the Value has been wrested from the House and all that remains will be mere scenery.
This was unfortunately prescient, as the cleverness of presenting the core labyrinth/mysteri as "a text to be interpreted" was excellent, but indeed once the puzzle's answer was revealed, it was not very satisfying to consider the continuation of any part of the story or setting.
The cover blurbs portray the book as a sparkling gem of prose, an all-absorbing world full of beauty that will irresistibly entice the reader, and a fascinating puzzle. I found the book to be well-written, but describing things as irresistibly beautiful and actually invoking the impression that they are beautiful are two different things. The book does a lot of telling (repeatedly, "The Beauty of the House is immeasurable") and not a lot of showing; mostly we come to understand, gradually, that the narrator's seeming-unreliability all has completely predictable causes, and actually that everything he reports is true. All mysteries are solved, every single clue that I noted was tied up neatly in the most obvious conclusion, there were zero twists and few reveals, no characters had any subterfuge or depth. Everyone was exactly as they seemed on the surface.
And finally, my observation of several books I've read recently: I get the impression that the book's conclusion was written under duress, with the publisher demanding "just finish the book and turn it in!", and not given any time to develop into something interesting or satisfying. The book definitely ends, and that is what can be said about it: in the last page there is a feeble grasp at Greater Meaning which falls completely flat and ends the book on a gaspingly sour note.
My recommendation: if you want an atmospheric book about a mysterious, endless house, read Mervyn Peake's Gormenghast. If you want a book told by an unreliable narrator, then I strongly recommend Tamsyn Muir's recent release Harrow the Ninth (and its preceding book, Gideon the Ninth). If you want a book that explores, in a fun novelistic way, the boundaries of human knowledge and the notion that modern scientific rationality has cut us off from access to certain domains of knowledge and maybe even certain actions and physically real spaces, then I recommend Neal Stephenson and Nicole Galland's The Rise and Fall of D.O.D.O. (which also includes an improvised modern Viking saga!). And if you want an excellent book written by Susanna Clarke, read Jonathan Strange & Mr Norrell.
This post's theme word is heterography (n), "a spelling different from the one currently in use." The vaguely timeless ambience would have been more interesting with heterography.