In Between the Worlds: Joan Lennon, Questors. London: Puffin, 2007.
Three children are plucked from their every day lives and take to The House in London. They find it's a "house between the worlds", and each of them has grown up in one of the worlds, but their mother is actually an agent of the house, although they have three different fathers. The three discover they were genetically engineered for a quest to save the universe (don't groan, it isn't written like that at all, and the quest has been engineered as well) but unknown to them their genetic engineering has been tampered with. The quest sends them back to each world in turn to find an object of power, and on the way they learn to see their own worlds through the eyes of others: the six nostrilled dragon is a delight and I couldn't resist a world in which "feral" warfare has been rejected in favour of "domesticated" warfare shaped by experience with crop rotation.
Oh yes, and while Bryn is a boy, and Madlen a girl, Cam is an it. On its world gender happens at puberty. What's the point of having gender earlier......?
It's well written in parts with the occassional clunky line. If I had to link it to any other books I'd say that Joan Lennon has been reading Isaac Asimov The End of Eternity and Diana Wynne Jones's Tale of Time City. Lennon is inventive rather than imaginative and she is very good at delivering some quite profound scientific ideas in among the jokes.
Not too sure of the age range for this book--maybe early teens with a lot for adult readers.