Best advice for learning about the foundations of Unix/Linux?

I’ve used all sorts of variants of *nixes over the years but I’ve really only ever learnt enough to get them to do what I need (and relied on people who know how to do the complicated stuff). I would really like to start learning about the foundations of the OS (stuff like how processes communicate with each other, that level) as I’m sure to learn a lot of stuff that interests me.

So basically I’m looking for advice on where to start but also perhaps any suggestions in terms of a distribution that would be possibly better geared towards this. I’m quite comfortable with users, groups, permissions and the general stuff. I’m not really interested in something like Ubuntu or Gentoo as they seem to be highly developed and, I suspect, much more complicated.