The Seal of Solomon (The Gifts of Solomon) (Volume 1)

That asshole of a head librarian Geoffrey made a huge mistake humiliating Darren in front of the entire staff. Darren's only fifteen, but he got his hands on some magic. He just wishes the magic came with a few disclaimers:

1. May spontaneously ignite gaslights
2. Ensure all nearby friends are properly secured
3. Possession by a powerful demon will occur, handle with care

The demon is a particular problem, especially when Darren's plan for revenge backfires and he loses control over the magic, landing one of the other apprentices in the infirmary. Unless Darren finds a way to tame the demon, it'll slowly consume him from the inside out.

Having the demon inside him also makes Darren one of the most powerful magicians in Balnibarbi. And according to the laws of magic, anyone who wants the demon's power just has to carve up Darren's heart.

Crossing paths with a power-hungry magician spells even more trouble for Darren and he learns magicians will burn and kill everything in their path to get what they want.

And what this magician wants is Darren's magic.