This is the second book in 'The Infernal Devices' trilogy, prequels to 'The Mortal Instruments' series, set in the same fictional world. The first book is Clockwork Angel, and the third is Clockwork Princess.
While the first book focused mainly on the mechanical creatures and the Will/Tessa romance, in this we see more of Jem. I didn't think much of him in CA, but this story made me feel more sympathetic to him and his illness. Tessa and Jem become VERY close, and thus the typical love triangle is established.
But it's a bit unorthodox because Will and Jem are the closest of parabatai, fighting partners sworn by blood oath, and love each other more than brothers. So there's no rivalry, thanks to that, and also because neither have any clue about the other's feelings. Which is a bit unlikely, that two people that close would never confide in one another that they were in love, or that neither would notice the other staring at the girl they love.
There's still a lot of action and typical Shadowhunter coolness, but there's more in here about the characters feelings and background stories. I began to care more for what happened to them, and saw them more favorably. If you've read Clockwork Angel, you should definitely buy this. It keeps up the pace, carrying on the storyline but adding thickness to the plot. A really interesting read, and one of a fab trilogy that is almost as good as TMI. But failing to meet TMI's standard was inevitable, as those books are a stroke of genius.