Impossible Creatures / Rundell, Katherine.

"A boy called Christopher is visiting his reclusive grandfather when he witnesses an avalanche of mythical creatures come tearing down the hill. This is how Christopher learns that his grandfather is the guardian of one of the ways between the non-magical world and a place called the Archipelago, a cluster of magical islands where all the creatures we tell of in myth live and breed and thrive alongside humans. They have been protected from being discovered for thousands of years; now, terrifyingly, the protection has worn thin, and creatures are breaking through. Then a girl, Mal, appears in Christopher's world. She is in possession of a flying coat, is being pursued by a killer and is herself in pursuit of a baby griffin. Mal, Christopher and the griffin embark on an urgent quest across the wild splendour of the Archipelago, where sphinxes hold secrets and centaurs do murder, to find the truth - with unimaginable consequences for both their worlds. Together the two must face the problem of power, and of knowledge, and of what love demands of us"--Publisher's description.