God of Clocks picks up soon after Iron Angel leaves off. It doesn't really add much that's new, though the battle continues, and some of the mysteries are revealed. I'm sorry to say that it's only some.
Campbell wove in a number of intriguing threads in books 1 and 2. He leaves quite a lot of them loose in this third volume. There are two other books in this universe, Lye Street and Damnation for Beginners, but they appear to be relatively free standing. With that, I'd have to count this closing book of the trilogy as a disappointment. Where Iron Angel fulfilled its promise in unexpected ways, this book does not. It moves the story forward, but deteriorates at the end into a not very satisfying dead end.
Towards the end of the book, Campbell inserts a time travel segment. (God of Clocks, after all.) As most time travel efforts do, even in defter hands, this ends with a whimper. Campbell pulls of a slight variation of the time travel standards, but only a slight one. And it ends poorly - more a colorful plot mechanism than credible narrative.
As noted, there are two other books in the universe, but I don't expect to buy either of them. I'd sum up this trilogy as interesting and innovative, but hampered by occasionally inadequate description, and brought down by a substandard ending. I can recommend the first book, Scar Night as a interesting read, but I'd stop there. You could even stop after book 2, Iron Angel. But in my view, this third book will frustrate and disappoint you.