‘That’s not how the text is supposed to go,’ said Emily.

Nick looked at the next picture. It was a lion, but not like the regal animals on the cards. It lay on its side with its crown askew. Its mangy fur had been pulled apart and a colony of maggots ate out its entrails. Its dull eyes lolled in its skull, almost as if it was still alive. A cloaked figure lurked behind it, watching, its face hidden in the shadows of its hood except for a row of giant serrated teeth.

Nick never forgot what he saw that night. It read like a monument to grotesque obsessions: bestial sex, deformed bodies, malice, torment and decay. Thanks to Bret, Nick had seen some of the most graphic images the Internet had to offer. Compared to that lurid realism, the black-and-white engravings in the book were plain, almost naïve. But even after five hundred years they maintained a savage power, a heightened truth in the anguished faces and debased bodies that shocked more viscerally than any photograph.

Each page brought new invented beasts: the monasticus, a double-jointed eunuch who spent all his time feverishly licking the scars where his genitals should have been; the equevore, a man with a horse’s head and a penis so large it required its own chain mail and helmet. A string of broken women lay behind him where he had raped them until they snapped in two. And in every picture, the cloaked figure, his savage teeth grinning in approval from under his hood.

On the penultimate page, a creature with a pig’s body and a man’s head, naked except for a hat, knelt on all fours. A dog in a crown squatted behind him and sodomised him, while another held him by his ears and thrust himself into the pig’s mouth. From the look of wild ecstasy contorting his puffed face, the pig was enjoying it. It was hard to tell if he was man or woman: he had a man’s genitals, but a woman’s breasts dangling from his distended belly to suckle the pack of wild men who bayed at his feet. Behind them all rose the cloaked figure, now swollen to three times his height, leering over them like a plume of smoke.

‘Who is that?’ said Nick.

‘The pig in the hat is the Pope,’ Emily said. ‘The dogs are the King of France and the Holy Roman Emperor of Germany. Knowing the date and the place, I’d guess it was intended as some sort of metaphor for the Armagnacken attacks of the 1440s and ’50s.’

‘What about the guy behind them?’

Gillian turned, her eyes shining with excitement. ‘Can’t you guess?’

The last animal in the bestiary was the rat. It seemed to have been added as an afterthought – the cloaked figure who haunted the book was absent from this page.

‘The rat follows the goose to its nest and murders its young.’

Beside the text was a scene of domestic devastation. The rat, wearing a square cloth cap like Fust’s, sat up on its haunches and tore the head off a downy gosling, still nestled in its egg. Its young eyes were wide with terror, staring at the mother goose, unable to understand why she did not come to help. The mother watched, helpless. Her wings had been ripped from her body and lay useless on the ground; blood poured from her breast where her heart had been gouged out. In her anguish she had not yet noticed. A rat pup with a face like Peter Schoeffer clung to her leg and gnawed it.

I confess, my first reaction was not outrage or a scandal; it was jealousy. While the quires of my Bible lan guished in the storeroom at the Humbrechthof, slowly mounting up, Kaspar had taken the first fruits of my creation. He had beaten me.

He watched me eagerly. ‘What do you think?’

‘It is…’ I slumped on the bed as the full enormity of what he had done struck home. ‘Obscene.’

‘But beautiful. Everything we dreamed of, before Fust tore it down.’ He knelt beside me and caressed the page. ‘My pictures and your words.’

‘Those are not my words.’

‘Our two arts fused as one. This is our master-piece.’ He pointed to the chapter heading. ‘I even managed to press the rubrics in red.’

I turned through the book. In one sense he was right: the book was immaculate. The proportions were pleasing, the pages precisely aligned: every drop of ink seemed to shine from the page. The illuminated images shimmered in gold, but it was the gloss of pure poison.

‘How many of these have you made?’

‘Thirty.’

‘Are they here?’

‘Not far away.’

‘Bring them to me,’ I commanded. ‘You must bring them back so they can be destroyed.’

The grin persisted, though strained. ‘Why should I destroy them? They are perfect?’

‘They are abominations,’ I cried. ‘You have taken everything about my art that was good or noble, that might have benefited the world’s salvation, and debased it. You are the tempter, the serpent in the garden.’

‘And you are a blind fool.’ In an instant a terrifying rage transformed his face. ‘A feeble-minded idiot who has stumbled on a power he does not understand. I have harnessed it to the one force in the world that deserves it.’

I sat on the bed, dumbfounded. In the silence between us, I heard the creak of footfalls on the stairs. We stared at the door, frozen in our battle like the beasts and hunters in the book.

Father Günther appeared on the landing. ‘Johann? It is almost eleven o’clock. They are waiting for you at the court.’

All my bones had turned to wax. ‘I cannot go.’

Günther stared between me and Kaspar, a witless spectator to our unfolding cataclysm.

‘You must go. Otherwise, they will make a summary judgment against you and you will lose everything.’

I fell back on the bed. The court, the judgment, Fust – they were nothing. Kaspar had unscrewed the form of my being and scattered the pieces. Everything in me, all that had meaning, was lost.

‘You and Keffer go. Report back what Fust says against me.’ He hesitated. ‘If you cannot answer him-’

‘Go.’

‘Are you ill? Perhaps I can persuade the court to delay.’ He glanced at Kaspar, imploring him for help. Kaspar played with the cover of his book and said nothing.

‘Leave me alone,’ I hissed. ‘It is done.’

With a last, bewildered glance at Kaspar, Günther hurried from the room. I heard his footsteps recede down the stairs, the bang of the door as he left the house.

Through tear-stained eyes, I looked up at Kaspar. I felt the vellum of his hateful book, smooth as a lamb.

‘All the things that Fust accuses me of: the missing parchment and ink, the types that reappeared in the wrong place. That was you.’

‘Some – not all. The priest Günther has had a profitable sideline of supplying the scriveners of Mainz with paper for the last year. And often at night when I crept down to use the press, I found Peter Schoeffer practising his craft. Perhaps he knew this day would come.’ He laughed at me. ‘You were always a poor judge of character, Johann.’

I gazed at him, trying to hold together the shattered pieces of my heart. ‘Why did you do this to me?’

‘I did it for you. To show you the potential of what you have created. In the same way as it took the serpent to free Adam from the garden of perfection where God held him captive, I wanted to make you see what could be done.’

He pointed to the bestiary he had given me in Strassburg. ‘Do you know how much that cost the man who commissioned it? Fifty gulden. And what is it but a mirror to flatter his vanities? I gave him what he paid for. But with your press, Johann, we can change the order of things.’

He touched the scars on his face. ‘You know how I got these. Because a king, an emperor and a pope – Christians all – raped their lands in the name of God. But in my torments, the Armagnaken taught me there are other powers that hold sway over this earth. I learned things from them – secrets that even the Church fears.’