He turned, and looked up into the face from his nightmares.

There were twelve pages, single columns of closely spaced writing. Over the centuries the catalogue had been subject to many revisions. Ruled lines eliminated many of the entries, while the margins had become a parallel narrative of names, dates and scribbled numbers.

‘What are we looking for?’

‘Whatever Gillian was looking for when she found the reference to the Devils’ Library. Probably something from the fifteenth century. Maybe a bestiary, or a title we haven’t heard of before.’

Nick tried deciphering the first line of the catalogue. Medieval handwriting and Latin defeated him.

‘How’s it organised?’

‘Chronologically.’

‘It shouldn’t be too hard to find the fifteenth-century books, then.’

‘Chronologically by the date that the monastery acquired the books,’ Emily corrected him.

Nick stared at her incredulously. ‘What sort of way to organise information is that?’

Emily laid a protective hand on the page. ‘Card catalogues weren’t invented until the eighteenth century. Until then, all they could do was write down the books as they came into the library. When they were sold, or lost, they crossed them out. You can see why librarians were so important to monasteries.’

She leaned over to examine the first page. ‘A lot of these books will have been Bibles or prayer books. We can probably discount those.’

‘Unless the monks pulled the same trick Gillian did here,’ Nick muttered. ‘If the book was so controversial, maybe they misfiled it.’ He pointed to an entry in the middle of the page. ‘What’s that?’

‘ITEM: DE NATURA RERUM.’

‘When does that date from?’

‘Originally? Probably about 300 BC.’ Emily couldn’t help smiling. ‘It’s Aristotle.’

Olaf hadn’t properly believed in God since he was fifteen. But he believed in destiny, and at his age he had seen enough to recognise it when it arrived. He didn’t resist Cardinal Nevado taking the handles of his wheelchair and wheeling him out. After the blue womb of the church, a foul yellow light drenched the world outside.

‘You hid yourself well,’ said the voice in his ear. ‘We have been trying to find you for a week.’

‘I always assumed you would.’

‘But you did not really believe it.’ The wheels skidded as he turned a corner. ‘For such a cunning race, humanity is badly adapted for deceit. We trust too much to experience. If we have got away with something once, we assume it will always be so. But the more often we risk something, the more likely we are to fail. You betrayed us once with the American girl and escaped. You should not have met in the same place a second time.’

Olaf stared ahead. ‘I like to look at the angels.’

‘I hope you enjoyed them. They will be the last you see for some time.’

Olaf suddenly became agitated. He craned around, but all he could see of his captor was a long black coat, a wall of darkness billowing behind him.

‘What do you want me to tell you?’

‘Nothing. We know everything.’

Cardinal Nevado reached in his pocket and pulled out a heavy fisherman’s knife. He hooked it around the wheelchair’s brake cord. With a twitch of his wrist, the blade severed it.

‘God pardon you whatever sins or faults you have committed,’ he murmured. He placed a gloved hand benevolently on Olaf’s head and held it there for a moment in blessing. Then he pushed.

The hill was steep and smooth as glass. Olaf flapped his arms and tried to grab the wheel rims, but on the hard snow it made no difference. It careered down the street and skidded out onto the road at the bottom – a two-lane stretch of ring road.

From the top of the hill, Nevado actually saw the wheelchair pop into the air as a truck hit it front on. It flew into the next lane, bounced, and disappeared under the wheels of an oncoming car.

There wasn’t much Nick could do. He paced the basement reading labels on boxes, wondering what other secrets they might hide. When that got boring, he turned on his laptop to play solitaire. When he’d lost three games in a row, he went back to Emily at the table.

‘Any luck?’

Emily frowned. ‘It’s hard to tell. A lot of these titles are unfamiliar. I can’t pin down dates at all.’

The clock on the computer showed half an hour had passed already. Without much hope, Nick activated the laptop’s wireless card. To his surprise, it found a network almost at once.

‘Maybe I can help.’

He dragged a cardboard box to the table and used it as a makeshift stool. As Emily read out titles he ran them through a search engine, gradually circling in on the section of the catalogue that seemed to have been written in the mid-fifteenth century. In the bottom corner of the screen, minutes ticked past.

‘Here’s one,’ she said. ‘No details at all, just a title. Liber Bonasi.’

Nick typed ‘Bonasi’ into the search engine.

Did you mean: Bonsai?

Underneath, a list of links offered him various services to do with bonsai trees.

‘Nothing coming up.’

‘Strange.’ She pored over the catalogue. ‘Try “Bonasus”. That might be a more common form.’

Japanese nurseries disappeared. Nick scrolled through the new results and tried to make sense of them.

‘Bonasus seems to be the Latin name for some kind of Polish bison. Apparently they piss on wild grass which they use to flavour vodka. Gross, huh?’

He looked up from the screen, wondering why she’d gone so still. ‘That’s what it says here.’

‘Try “Bonnacon”.’

She spelled it out for him. Nick entered it and clicked on the first result that came up. Emily stood and walked round the table to look.

‘That’s it.’

A new picture filled the screen: a strange beast with muscular flanks, curled horns and cloven hoofs. Three knights were pursuing it with spears, but had been thrown back by a forked blast that shot out from the beast’s rear end like lightning.

Nick read the caption. ‘Bonnacon, also known as Bonasus. A fabulous beast with a horse’s mane, a bull’s body and horns that curve back and so are useless for fighting. When attacked, it runs away, while emitting a trail of dung that can cover two miles. Contact with the dung burns its attackers like fire.’

Half-listening, Emily pulled the reassembled printout from the bag and laid it next to the computer. Nick looked between them: screen to paper, paper to screen.

The animals were the same. Not identical, but clearly the same fantastical species. What Nick had taken for a bushy tail was actually a cloud of fiery excrement spraying behind it.

‘Napalm shit,’ said Nick. ‘Glad I never walked behind one.’

‘Liber Bonasi means the Book of Bonasus. It could be a pseudonym, like Libellus or Master Francis.’

Nick moved around the cluttered table to look at the catalogue again. ‘So does it mention the Devils’ Library there?’

‘No.’ Emily pointed to the entry. The title had been ruled through, struck out, but unlike the other books there was no date or description of where it had gone. The margin was empty.

‘Olaf said Gillian found the reference here.’

‘If you were going to send a dangerous book to a secret library, you probably wouldn’t record it in the catalogue.’

‘Not so people could see.’ Nick pulled out his cellphone and turned it on – the first time he’d touched it since he left Paris. Blue-white light glowed from the screen.

‘It’s not ultraviolet, but it might give us an idea.’

He laid the phone flat against the medieval catalogue. Light spilled across the page. With tiny movements, he angled the phone back and forth, trying to catch any sign of hard-point writing.

‘What’s that?’

He just caught it: a faint scar in the paper, almost invisible in the weak light. He turned up the brightness on the screen, tracing the indentations like an archaeologist sifting through sand. He had to spell it out letter by letter; several times he realised he’d got one wrong and had to go back.