“Very well,” said Nightingale. “We’ll leave all the mundane connections in the capable hands of the detective sergeant. I think our first move should be to track down the source of the book.”

“I figured Dunlop stole it from the Bodleian Library,” I said.

“That’s why you shouldn’t make assumptions,” said Nightingale. “This is an old book. It could have been stolen prior to Dunlop arriving at Oxford and then come into his possession by some other route. Perhaps the person who trained him.”

“Assuming he was a practitioner,” I said.

Nightingale tapped his butter knife on the plastic-wrapped copy of the Principia Artes Magicis. “Nobody carries this book by accident,” he said. “Besides, I recognize the other library mark. It’s from my old school.”

“Hogwarts?” I asked.

“I really wish you wouldn’t call it that,” he said. “We can drive up to Oxford this morning.”

“You’re coming with me?” Dr. Walid had been very clear about the whole taking-it-easy thing.

“You won’t get access to the library without me,” he said. “And it’s time that I started introducing you to people connected to the art.”

“I thought you were the last?”

“There’s more to life than just London,” said Nightingale.

“People keep saying that,” I said. “But I’ve never actually seen any proof.”

“We can take the dog,” he said. “He’ll enjoy the fresh air.”

“We won’t,” I said. “Not if we take the dog.”

FORTUNATELY, DESPITE the overcast, the day was warm, so we could head up the A40 with the windows down to let out the smell. Truth be told the Jag isn’t that comfortable as a motorway car, but there was no way I was heading into a rival jurisdiction in the Ford Asbo—standards have to be maintained, even with Toby in the backseat.

“If Jason Dunlop was trained,” I said as we climbed on to the Great West Road, “then who was his teacher?”

We’d discussed this before. Nightingale said it was impossible to pick up organized “Newtonian” magic on your own. Without someone to teach you the difference, vestigia are hard to distinguish from the random background noise of your own brain. The same was true of the forma; Nightingale always had to demonstrate the form to me before I could learn it. To teach them to yourself you’d have to be the kind of insane monomaniac who’d deform his own eyeball to test his theories on optics—in short, someone like Isaac Newton.

“I don’t know,” said Nightingale. “After the war there weren’t that many of us left.”

“That should narrow down the suspects,” I said.

“Most of the survivors would be very old by now,” said Nightingale.

“What about other countries?” I asked.