This is a short book, some 250 pages long and very accessible. That's about all I can come up with to describe it so I'll quote directly from Amazon :
Piranesi lives in the House. Perhaps he always has.
In his notebooks, day after day, he makes a clear and careful record of its wonders: the labyrinth of halls, the thousands upon thousands of statues, the tides that thunder up staircases, the clouds that move in slow procession through the upper halls. On Tuesdays and Fridays Piranesi sees his friend, the Other. At other times he brings tributes of food to the Dead. But mostly, he is alone. Messages begin to appear, scratched out in chalk on the pavements. There is someone new in the House. But who are they and what do they want? Are they a friend or do they bring destruction and madness as the Other claims?
Lost texts must be found; secrets must be uncovered. The world that Piranesi thought he knew is becoming strange and dangerous.
The Beauty of the House is immeasurable; its Kindness infinite.
I have to say this is the most original story I've read in a very long time. Well written, atmospheric and keeps the reader guessing even after the end - furthermore it's not hard to read!
ETA There really was a man called Piranesi, famous, for among other things, etchings of imaginary prisons of which there are 16 - 16 is a significant number in the book