‘Malware,’ said Nick. ‘The book’s a virus. The press spreads it quickly – much faster than before. People read it and get infected. Eventually you end up with a whole network of infected people who you can use to launch attacks.’

‘The Reformation,’ said Emily.

‘I doubt that Pope Pius would have thought of it so – but yes. Truly, there is nothing in the world the Church has not seen before. Pius knew that if Gutenberg’s monstrosity became known, the printing press would have been condemned as an agency of the devil. He suppressed all trace of the Liber Bonasi and left a decree that every copy should be wiped from the earth. Thirty copies were made. One remains here as an exemplar. Twenty-eight more have been hunted down over the centuries, dug out of the libraries and collections where they lay buried, and destroyed. Only one remains outstanding. And now you have brought it to me.’

Nick was feeling faint. He looked up to try to clear his head, but the towers of books looming into the darkness only made it worse.

‘Why do you even bother?’ said Emily. ‘Gutenberg, the Master of the Playing Cards, whoever made that book: they won. Any worthwhile technology can be used the wrong way. However many copies of the bestiary came off that press, you’ve still printed more Bibles. Isn’t that a better trade-off?’

For the first time, Nevado looked angry. His ageless face suddenly became old. ‘This is an ancient war between good and evil. You cannot compromise with Satan. Pope Pius was wrong. The Church was never stronger than when books were rare and costly, written individually in a language only a learned fraternity could understand. To keep these books here was nursing a serpent in our breast. They should have been destroyed.’

‘I never knew the Church was so squeamish about burning books.’

The anger ebbed. The blood-red lips twisted into a cruel smirk. ‘Everything in its time. Why do you think I suffered you to come here?’

The adrenalin was running out. Nick could feel the crash coming. ‘We broke in.’

‘Why do you think you found the hidden map, the ladder leading you into the tower? Did you think we are so trapped in the Middle Ages that we do not even know how to lock a door?’

‘Wouldn’t surprise me,’ Nick muttered.

‘This moment, with Pope Pius’s charge at last complete, is a fitting time to end his folly. The library will burn, and you will burn with it. They will find your bones in the ashes and you will be held responsible.’

‘Why not just do it yourself?’

Nevado held up his hands. His skin was parchment thin, veins like rivers just below the surface, but they were steady as ice. ‘You think I am old and feeble? I have achieved much, but I have not finished my journey. I still have ambitions.’

‘Will letting a priceless collection of books burn help you become pope?’

‘Few cardinals in the conclave will ever know of it. Those who do, most of them will be glad it has happened. They will hear that a gang of international art thieves broke into the library to steal the manuscripts, overpowered the monks and the guards and could not be stopped. In their greed they grew careless. They dropped a cigarette; papers caught light; the library was lost. They were caught in the fire and burned almost beyond recognition.’

‘And we’re supposed to be a gang of international art thieves?’

‘Why not? A man wanted for murder in New York: a computer expert who could disable our security systems. A medieval scholar with a known animus against the Church. And a disgraced auctioneer who stole from the properties she was supposed to be valuing. You came here of your own will, following your own trail of evidence.’

‘For someone who wanted us to come here you spent a lot of time trying to kill us.’

‘I was over-hasty. You would have been killed in New York if my associates had managed it, or Paris or Brussels or Strasbourg. Always, you escaped. I wondered how you could prevail against forces so much greater than your own; I prayed God to deliver you into my power. Finally I understood. He has brought you here to bring me the book and fulfil my purpose. His purpose. Truly, He moves in mysterious ways.’

He took a cigarette out of his coat and lit it. A nostalgic smile spread across his face as he took a drag. ‘I quit fifteen years ago. As my doctor said: they will kill you.’

‘There’s only one problem,’ said Emily. ‘You’ve got the wrong book.’

‘Where are the rest of these books?’

Always the same voice. Always the same questions. I longed to answer but I could not. A crushing weight bore down on me. It milled my wretched body, choking my lungs, bending my bones until they snapped.

‘I don’t know.’

I did not know anything. Where I was. How long I had been there. Who held me captive, and how they had come by the book. All I knew inside my sackcloth hood was the rattle of chains, the smell of wet stone and burning pitch, the ceaseless questions I could not answer.

I was naked – I knew that – tied to a frame like parchment being stretched to dry. A flat board rested on my stomach, held down by a great and increasing number of stones. It was an exquisitely apt punishment – that I who had devoted myself to pressing ink, lead and paper should now go under the press myself. I wondered if Fust had told them.

‘Men speak of the new art you have discovered. Was this what you intended? A tool for heretics?’

‘I wanted to perfect the world.’ It had seemed so vital to me, a burning purpose. Now it sounded feeble.

‘Did you seek to destroy the Church?’

‘To strengthen it.’

‘To summon the powers of darkness?’

‘To spread truth.’

The inquisitor leaned over me. I knew, because I could smell the onions on his breath. Air fanned my neck as he waved something – the book? – in front of me.

‘Is this what you call truth? The most diabolical lies and filthy slanders that the devil ever planted? Even to look on this book would be mortal sin.’

My chest burned. ‘I did not make the book,’ I gurgled.

He ignored me; he always did. The pain of torture might break a man’s body, but it was the futility that destroyed his soul. The questions never changed; the answers were never believed.

‘How many did you write?’

‘Thirty.’ I spoke eagerly, almost grateful for the chance to answer his question. ‘He said there were thirty.’

‘One was sent with an obscene note to the archbishop. Another was found on the step of St Quintin’s church – a perfect copy. Is that the devil’s work?’

‘My art,’ I gasped.

‘So you confess?’

Panic gripped me. Had I confessed? I tried to explain; I heaved against the board to get air in my lungs, but all I managed was a strangled groan. Then I realised how ridiculous it was and lay back. I could not condemn myself any more than they already had. I would die there.

I heard a grim laugh. ‘You will not die here.’

I must have spoken aloud.

‘When we have learned what we need, we will burn you in Mainz as a heretic.’

A small sigh escaped my body, perhaps the last breath in me. It was the end I had always known would come, the lesson my father had tried to beat into me that day in Frankfurt. I would die a heretic, a forger who had debased his currency.

Despite everything, I found myself laughing: the mad cackle of my rotten soul fleeing. I had lived half my life haunted by fear of burning for the mortal sins I had committed against body and nature. Now I would burn for a book I had not made. I suppose it was a sort of justice.

My laughter enraged the inquisitor. He shouted to his assistants. I heard the grate of stone, and two ribs cracking as the weight bore down.

‘Where are the rest of these books?’

The pain consumed me, pressing me into oblivion.

For a second, Nevado was absolutely still. Then he pushed past them and strode to the shelves at the back of the room. The gunman by the door edged closer.