Jake Adelstein wrote a book about being undercover with the Japanese Yakuza, it's called Tokyo Vice. It's a little sensationalised but overall a good account of someone undercover in a dangerous context. There's a much better investigative book by David E Kaplan (Yakuza) but that one is less about BEING undercover than demonstrating all the dirt you can dig up on the Yakuza if you prod the right people, so I'm not sure that would be helpful for you.

Like navaan says, there's Günter Wallraff, but I have no idea if any of his research is available in English.