Knights of the Borrowed Dark by Dave Rudden

The first book in a new series about an orphan boy who discovers he is part of a secret army that protects the world from a race of shadowy monsters.

Grey placed his finger in the middle of the shadow.
'What's this?' he asked.
Denizen frowned. 'It's a shadow.'
'No, it isn't,' Grey said. 'It's a door.'

Denizen Hardwick doesn't believe in magic - until he's ambushed by a monster created from shadows and sees it destroyed by a word made of sunlight.

That kind of thing can really change your perspective.

Now Denizen is about to discover that there's a world beyond the one he knows. A world of living darkness where an unseen enemy awaits.

Fortunately for humanity, between us and the shadows stand the Knights of the Borrowed Dark.

Unfortunately for Denizen, he's one of them . . .