The problem here might be trying to cram too much into too short a book. (t's actually 288 pages; there's an excerpt from The Goblin King adding to the page count). The end result is that the story keeps getting interrupted by just enough world building to be dangerous.
How dangerous, you ask... Ah. Well, the fairies are not ah... well... it's odd really. it seems that a male and female fairy are not usually capable of reproducing, so the usual thing is to seduce a human into a union. Except... apparently this is forbidden. Yeah. Ok. The H is the result of such a union. And here we get tricksy. Born in the mortal world, the person is mortal and more or less human with quirks (think sidhe-seer, among other things) Born in Annwyn... fairy. Maybe it's safer to say the fae are only fertile on the mortal plane.
I'd also add that this one monkeys a bit with the concepts of heaven and hell. Personally, i'd rather authors just deviated entirely rather than redefined - not really liking the thought that the decided amoral fae are sort of like angels (author doesn't actually SAY that but she had to know the mind would go there)