Basically the idea is that there are multiple words next to each other all accessable through points they touch, much like the idea of parallel dimensions and the like. By singing, dancing, and drumming you can create certain patterns that allow you to fall down the rabbit hole into another world. Everyone has a 'home' dimension where they're born and everywhere else, they're 'strangers'. Strangers have certain abilities that aren't avaliable to the natives, namely that of charisma and an ability to accumilate mana. Mana comes through worship and sarifice and can be used to do all sorts of things from influencing other individuals to casting lightning bolts.
What the books basically provide is an explanation for God, of sorts. In the alternate world of 'Nextdoor', there are more Gods than you can shake a stick at, each having staked out a portfolio over which they rule and derive their power. A God of Knowledge would have his priests stockpile books and information, doling it out in exchange for offerings and whatnot. Each person that believes in the God provides him with mana with which he can influence more minds or cause miracles. It's sort of like a pyramid scheme of sorts. The big Gods will go out and hire on people to play lesser Gods and the lesser Gods will send a portion of the mana they receive up the ladder.
Anyway, the reason I wanted to read the books again is because I always sorta liked this idea of faith providing a real manifestation of power. It's not stated explicitly in the books, but it's not hard to chalk Jesus or Muhammad or Moses up as just another Stranger from a different world that came here and set up shop. I'm not sure if it was Duncan's intent to take God down a notch or two, but I wouldn't put it pass him. He is canadian after all. As for me, I just just it from a more metaphysical point of view. If faith were a measurable quantity and could actually be used to perform miracles or affect the environment, then the possibilities would be endless. Frankly, I think I'd find it to be a mroe comforting way to think about God if it were true. At least then you can sort of understand the equation instead of pondering the unknowables.
Besides, I wouldn't mind treking through a wardrobe and setting myself up with a slice of divinity. I think I'd make a pretty spiffy God overall. I would watch over my people and see that they prosper. Well, until they piss me off at which point I'd probaly smite a whole bunch of them. But they'd deserve it. Who are they to fuck with God anyway?