I think that much of early Gentoo ideas - the OpenRC init system, the ports system, the handbooks - were inspired by BSD traditions. OpenBSD and FreeBSD each have good documentation. Working through the FreeBSD handbook, and then study of McKusik's BSD Book [0], are a good way to get another perspective if you get into this sort of thing.

[0] http://www.worldcat.org/title/design-and-implementation-of-t...