«Ink and Bone» by Rachel Caine

In 48 AD, a fire set by the troops of Julius Caesar destroyed much of the Great Library of Alexandria. But what if the fire had been stopped? What would the Library have become? Fast forward: the Great Library is now a separate country, protected by its own standing army. Jess Brightwell, seventeen years old and very smart, with a gift for mechanical engineering, has been sent into the Great Library as a spy for his criminal family…