Really good books about a retired CIA assassin, now doing contract work, in his 50's. Highly dangerous, he's a half American half Japanese who altered his looks to appear more Japanese so he could live in Japan more easily. Part of what's so cool about the books is that Eisler is VERY familiar with the parts of Japan he writes about in particular, but also other locations he includes. Pretty good martial arts writing to boot. He goes into a lot of detail about the painstaking methods it takes to live anonymously. Stories take place around the globe.
Highly recommended series.

There was a movie made a couple of years ago in Japan about the character that even had Gary Oldman in a key role. I couldn't finish it though, didn't seem presented right.

The good news:
Rain is coming to tv! Not sure if it's a mini-series or a regular show.

The bad news:
Keanu Reeves is playing him.

Might not be terrible, the guy is quiet and reserved, which Keanu does well. Of course, he isn't Japanese or look it, like 47 Ronin. I don't know why they keep trying to cast him that way?
At least he brings star power, so maybe this will be well done.

Yeah I've read all the John Rain books and, apart from a bit of clunky writing occasionally, hugely enjoyed them.
As you said, he really does know Japan, judging from his descriptions and locations. The series gets a lot more nuanced as it goes on, but still retains the best written fight scenes I've ever come across. I'm a bug fan of all the tradecraft he goes into, too.