FreeBSD sh(1) (not bash(1)) man page. That's just how I understood how to shell. Nowadays I'm running Debian and my $SHELL is /bin/bash, but when I was on FreeBSD I really learnt tools like make(1), sh(1); the man pages were pieces of art. Having read sh(1), I do have a nice grasp of how shell works in general, to which knowledge I can add anytime the higher-level goodies bash has to offer (though I generally prefer keeping it POSIX, and using an actual programming language when it doesn't cut it).