Behave by Robert Sapolsky. It’s an excellent book if you’re trying to understand why the people around you are like… well, that way. But if you didn’t study neuroscience, you’ll likely have to re-read it to get at least the most important points to stick.
I’ve read Suicide Island a while back until I was up-to-date at the time, that fits the bill. It’s by Kouji Mori, who was a close friend of Kentaro Miura and is now involved with the continuation of Berserk as far as I know.
Besides that, I probably don’t need to tell you that Vagabond and Vinland Saga are excellent. Lone Wolf and Cub is also really good in my opinion, albeit pretty old by now.
Yea, I really liked these as a kid. Shame their chocolate is collected by slaves :|