Can anyone recommend a modern explanation of sed's more advanced features?

I'm ashamed that, despite having learned many other ostensibly more complex systems and languages, sed continues to elude me. Of course I get the usual 's/foo/bar/g' stuff with regular expressions and placeholders, but I still don't really understand most of the other commands or, more usefully, the addresses.

Perhaps I need to spend quality time with the manpage in a cabin in the woods, away from modern distractions, but so far it has proven impermeable.

Perhaps I'm missing a proper understanding of its 'pattern space' vs 'hold space'.