Ironically enough, I was captivated by this strange, beguiling, beautiful tale of a man trapped in a fantasy domain. As with Clarke's brilliant Jonathan Strange & Mr Norrell, the magical fancy is fused with the entirely mundane, so that even the most outlandish elements feel credible.
One particular joy is that we're sometimes ahead of our narrator, who can be slow to make sense of the evidence presented. When he scoffs at such ridiculousness as "Manchester" and "police stations", we know he's missed something important - and true. I think that then prompts us to read his findings extra carefully, sifting for additional clues. We become active participants in the tale.
It's difficult to say more without giving away some of the mystery - and if you've not read the novel, then stop now.
I think it's brilliant that the ending is not about some lost eden, forever out of grasp. Instead, Piranesi - if he is still Piranesi - is helped by an amazing character to take charge of all that has happened, and then he helps others do the same. Among the literary and scholarly references, on page 165 there's mention of "Timey-Wimey: Steven Moffat [and] Blink", and there's the same satisfying intricacy and resolution. As with Blink, there's violence and loss, but what could so easily be (effective, moving) tragedy is in fact a joyous liberation. It's beautifully, deftly done - this whole puzzlebox of a book deceptively simple, and perfect.