This is a strange and strangely beautiful novel, but it didn't really grab me.

The tight first-person narrative introduces us to the title character, whom we eventually discover is a young man. He lives in a weird environment that seems to be a building of at least the size of a city, which he calls the House.

This House has galleries, grand halls and long hallways, with sweeping staircases that connect the three levels. On the bottom is the sea, complete with waves, currents, and wildlife of many species. The middle level has most of the architectural features and art: thousands of statues of many different subjects, including portraits and allegories. The top level (still inside the building, for the most part) includes birds and clouds that fly and float through the halls and galleries. The bottom level provides Piranesi with most of his food (fish, seaweeds) and also most of the danger in his otherwise uneventful life: on some occasions, the different tides of the oceans coincide and wash high up into the second level, flooding portions of it and dragging off anything that's not fastened down.

Piranesi loves the House, which he treats as his alma mater, his nurturing parent, and also rather as a god. He mentions repeatedly that it provides him with everything he needs and that he is grateful to be allowed to live among such beauty and wonder. He spends whatever time he doesn't need for subsistence activities in exploring and documenting his world. Although he is dressed in rags and fish skin and tattered sneakers, Piranesi has access to notebooks and pens. And occasionally, he has a companion, whom he calls the Other.

This is a well-dressed, well-fed man who visits regularly and sometimes brings Piranesi new sneakers, notebooks and pens, and food for which the narrator is grateful, considering these meager gifts to be part of the beneficence of the House, The Other takes his own notes about Piranesi on a mysterious, perhaps magical device, because Piranesi is assisting the Other in his quest for knowledge supposedly hidden in the House.

This weird and tenuous situation starts to unravel with the arrival of a mysterious Interloper, whom the Other believes to be dangerous and evil. Piranesi, hungry for human contact, at first believes the Other's warnings but soon begins to long to find this additional person. He also starts to examine his own records in more detail and is horrified to find that they are not nearly as accurate or coherent as they should be. Slowly, the answers to many of these mysteries are teased out — but not all.

The voice of the narrator nags at something half-hidden in my own memory: a science fiction story (probably a work much shorter than a novel) involving an aquatic human being, perhaps of very small size, who is being studied by some seemingly omniscient being. The line that sticks with me is "I must mate with Diane every day when the top of the water is brightest." For a while, I was convinced that the story must be James Blish' story "Surface Tension," but that is available online now, and it's definitely not the work I recall. The story I remember had very much Piranesi's voice, the same naïve narration from a being who assumes that his artificial environment is the world and that his orders come from a godlike source.

This faulty partial memory (and ironically, that's a theme of this book as well) certainly influenced how I feel about Piranesi. The novel is aesthetic in the extreme, beautifully and lucidly written, but it left me cold, emotionally, except for a certain amount of anger on the behalf of the narrator as I became aware of how he was being abused. An odd book.