Impossible Creatures by Katherine Rundell

The day Christopher saved a drowning baby griffin from a hidden lake would change his life forever. It’s the day he learned about the Archipelago, a cluster of unmapped islands where magical creatures of every kind have thrived for thousands of years—until now. And it’s the day he met Mal, a girl on the run who desperately needs his help.

Mal and Christopher embark on a wild adventure, racing from island to island, searching for someone who can explain why the magic is fading and why magical creatures are suddenly dying. They consult sphinxes, battle kraken, and negotiate with dragons. But the closer they get to the dark truth of what’s happening, the clearer it becomes: No one else can fix this. If the Archipelago is to be saved, Mal and Christopher will have to do it themselves.