I'm genuinely stumped on this one. A quick search for 'Tokyo book' yields too many generic results - there's no single famous book with that exact title that comes to mind. It might be a shorthand or a misremembered title. The one I've seen pop up occasionally in discussions is 'Tokyo Ueno Station' by Yu Miri. That's a deeply melancholic novel about a ghost haunting the park, reflecting on the disparity between the shiny, modern Tokyo and the invisible poor who built it. It's less a plot-driven story and more a lyrical, haunting monologue about memory, class, and displacement.
If it's not that, could it be 'The Tokyo Zodiac Murders'? That's a classic locked-room mystery from Japan's Golden Age of crime fiction, with a really intricate, puzzle-like plot about an artist's bizarre murder and a complex alibi. The main drive is solving this seemingly impossible crime. Or maybe it's Haruki Murakami's 'After Dark', which follows interconnected characters through one night in Tokyo, but that's more of a mood piece than a traditional plot. Without the exact author, it's tough to pin down.
I'd need more to go on. Was it a thriller? A literary novel? Sometimes people mean '1Q84', which has major sections set in Tokyo, but that's a whole different beast. Honestly, the phrasing 'Tokyo book' feels like something a friend would say when they can't recall the name but remember the setting vividly.