The Truth Sayer by Sally Prue
Oxford University Press ISBN 9780192754400
Nian is a boy with a destiny. Taken from his home and family to live in the House of Truth, he must practise his skills of mind-reading, weather lore and manipulation of matter. Once he has achieved mastery, he will become one of the elect, a Lord of Truth.
There's only one problem. Nian doesn't want to become a Lord of Truth. He just wants to get away. But his only means of escape is to step into another world. To be precise, into Jacob's front room. In Essex. Just before tea time. Nian doesn't speak English, he doesn't know what to do with a toaster, and he's got no idea what those roaring lumps of metal speeding down the road are. The truth is, this is going to be interesting....