So I've heard a lot of criticism levelled at Age of Sigmar's background, generally that it's a bit on the shallow side. But I've also heard a few people congratulate it on being quite an original concept. I'm interested, but I'm also not sure where to start digging into it.

Is it just the big rulebook that covers the world, or are there other ways to learn more about it? Do the Black Library books give a good grounding in the new realms? I'm a little cautious about picking up the main rulebook, as it's a hefty price for someone who's just curious.