This is one of those books that's unlike any other. It's surreal and dreamy and the sheer "what the heck's going on?" factor compelled me to read it all in one day.
A novel like this - light on plot, with an extremely limited cast of characters, told in an epistolary style - really sinks or swims on the narrative voice. Luckily the titular Piranesi is fun to read, and comes across as practical and clever, curious and sweet. His ignorance is charming rather than frustrating, and of course his naivete is all part of the mystery.
Highly recommended to anyone who loves an atmospheric and/or experimental story.