Had to say, sincce I'm both historically inclined and a swede, that Yggdra seems derivative of Yggrasil, the world tree with tree levels - asgard and midgard are on one, outgard is the second and the third being Hel, the underworld - of norse mythology.
I think the book in ratcatchers apperance is a reference to the fairytale of the ratcatcher. It's a classic Grimm brothers childrens story, about a city with a rat problem.