The Forever Court by Dave Rudden

Life is returning to normal for Denizen Hardwick. Well, the new normal, where he has to battle monsters in quiet Dublin bookshops and constantly struggle to contain the new powers he has been given by Mercy, the daughter of the Endless King. But Denizen may need those powers sooner than he thinks – not only are the Tenebrous stirring again but the Order of the Borrowed Dark face a new threat from much closer to home.