Ace in the Hole (The Dawson Sisters Trilogy) by Dallas Tillman

Evan's potential with dark magic but lack of ambition and drive leads him to an apprenticeship with the newest Master-class magician: Austin "Ace" Honoran. Ace is also a stage magician, and after a compulsion to obey his new teacher is installed with magic, Evan is whisked away into a world of dazzling lights and entrancing performances with his charismatic but taciturn companion. When an old friend contacts Ace, Evan assumes it's just going to be business as usual, but the old friend has something more in mind for the magician's dark magic talents, an offer that puts both of them in more danger than Evan expected to find. Evan must face choices of loyalty and compulsion, desire and reason, and complacency or mastery if he plans to get himself out alive and in touch with his sanity, for he has to help himself before he can help the one person he might actually consider his closest friend.