Okay, when I put it that way, you reply (a) "You read the mythology, it's all in the three earlier books" and (b) "Doesn't all high fantasy do this? Drop you into an unfamiliar mythology?"
Yes and yes. But Enge is playing the game way farther into the outfield than I'm used to. Not only are the cultural assumptions obscure, but the story beats that he's hitting with those cultural assumptions are obscure. It's that "I can tell this is an in-joke, but I don't know what about" feeling.
On top of that, dwarf society is taciturn -- this is where Morlock gets his habit of talking in grunts -- and then half the characters spend half the book being mind-controlled by dragons. So there's a very great deal of people having silent realizations which turn out to be lies. Infer cultural assumptions from that. I'm not saying I don't enjoy the challenge, mind you.
In frame, we also get a little more detail about the multiverse: Merlin is the Merlin from our world, for example. Good to know. Also, Enge read my previous reviews and explains the business of the sun coming up in the west. Okay, not so much "explains" as "offers a steely-eyed line of bafflegab designed to make me shut up and enjoy the goddamn story already", which, fair enough.