He doesn't even look up, explaining distractedly, "We were apprenticed together, as boys."

There is so, so much more to that story; love, trust, betrayal, war and revenge. Their apprenticeship didn't end well, in either of their parallel worlds. If she's good at reading people, there's a wealth of things he's not saying, and a lot of emotion attached to them, but he's trying to make it all sound offhand and unremarkable. He doesn't know her well, and doesn't know quite what Balthazar would trust her with. The basics are safe enough, though. They were both Merlin's apprentices, and what more does she need to know?

After a moment he shuts the book and vanishes it away with a vague wave of his hand. "The Encantus isn't saying much about current events." He levers himself to his feet stiffly, ready to try the tracking spell now.