Luke gave a great overview already...I'll just add a couple of my own thoughts...
I am probably not as into E as Luke...although I admit for some reason I haven't spent a ton of time with it. Of the more recent period, I particularly enjoy Axioma Ethica Odini and In Times.
But yes, Monumension is where they started getting "weird" (in a good way!
). One could argue the previous album, Mardraum was showing some signs already
There's plenty to digest in that period, and most of it is very cool/interesting. In that early-to-mid 2000 run, I have a soft spot for Isa but, again, it's all good.
As Luke says, before Monumension/Mardraum, it really was Viking themed black metal, but I think they were already doing it their own way. I always enjoyed Frost, and if you want an example of an ambitious tune from that earlier period, try the opener of the Eld album, "793 (Slaget om Lindisfarne)", over sixteen minutes of truly epic stuff. Following that, the Blodhemn album is one of their most aggressive...almost thrashy at times.
Let us know how your exploration goes!