The waiter thought for a moment. ‘We have the Castle Wolfsschlucht. But this is closed.’

‘You mean for the winter?’

‘For all of the time. Private. I think it is owned by an American.’

He put his hands on his hips and scanned the bookshelves over their heads. Nick and Emily waited. Eventually he reached down an old book with a frayed cloth cover and dog-eared pages. Beautiful Oberwinter, said the title.

‘If you want to know more, maybe this has something.’ Nick thumbed through the book while Emily wolfed down her food.

‘Here we are: “Castle Wolfsschlucht”.’

In medieval times, the building was a monastery dedicated to the Virgin Mary. Tradition says it was built on the site of a local shrine, though this has never been proved. The monastery was obedient to the Archdiocese of Mainz, with one of the most famous libraries in Germany. Most such foundations were dissolved during the Protestant Reformation of the sixteenth century, but the monastery petitioned the Emperor Charles V and was declared reichsfrei, independent of all local authority and answerable directly to the emperor himself. Residents of Oberwinter are still proud of the tradition that the pope himself interceded with the emperor on behalf of their monks, promising Charles support in his war against France in return.

The monastery was finally dissolved by the Secularisation Law of 1802. The title passed to the Counts of Schoenberg, who converted the buildings into a castle. In fact, the monastery was perfectly suited to this role. It is built on top of a steep rock overlooking the Rhine, surrounded on three sides by the Wolfsschlucht or ‘wolf’s gorge’. When the armies of Napoleon marched through the Rhineland, they did not even attempt to capture it.

In 1947 the castle was sold to an anonymous benefactor. It is closed to the public, but one can still glimpse it from the river and wonder what history lies behind those ancient walls.

‘The last sentence sounds rather plaintive,’ Emily said. ‘As if the author was almost as curious as we are.’

‘It is a bad place.’ The waiter had returned with another basket of bread. He lowered his voice, and looked around the empty restaurant for theatrical effect. ‘My grandfather has once told me that in the wartime, the Nazis are going there often. Always in the middle of the night. He said that even maybe the Reichsminister, Joseph Goebbels, has been there.’

Nick was about to ask him how Goebbels had got in, but at that moment a woman’s voice called the waiter into the kitchen. He excused himself. Nick looked back to the book.

‘There’s a picture.’

He spread the book flat on the table and turned it to face Emily. The image was dark and vivid: a lonely castle perched on an outcrop in a gorge between two mountains. Heavy lines scratched out a brooding sky, while a black river boiled in the foreground.

‘Is that where we have to go?’

‘If that’s where Gillian went.’ Nick laid the sketch map beside it. It was hard to see in the woodcut exactly how the castle was laid out, but there were two turrets that might correspond to the corners of Gillian’s pentagon, and a squat tower rising from the back that could be the keep. He rotated the drawing until it looked right.

‘Must be it.’

‘It certainly sounds as though the Pope went to a lot of trouble to keep it safe. There must be something in there he didn’t want the Protestant reformers finding.’

‘Or the Nazis.’

Emily studied the plan. ‘So how did she get in?’

‘This tower here’ – Nick indicated the X – ‘around the back. Gillian must have found an entrance there.’

Emily read over Gillian’s list in the margin. ‘Or tunnelled her way in with her spade and her head torch.’

‘Maybe we can improvise.’ Nick signalled for the bill. When the waiter came, he said, ‘Our car’s stuck in the snow just outside town. I don’t suppose you have a shovel and a piece of rope we could borrow to get ourselves out?’

The waiter looked surprised, but he was too polite to question Nick’s priorities. He went outside, and came back a few minutes later with a garden spade, a torch and a length of blue nylon rope.

‘Perfect.’

Nick paid the bill with the last of his euros. He felt bad that he couldn’t leave a better tip. He put on his coat and picked up the spade.

The waiter held open the door as they left. Snowflakes blew across the floor on a gust of wind; the glasses on the table rattled. The waiter peered out into the dark street.

‘Good luck.’

LXXVIII

Frankfurt, October 1454

When we revisit places from our childhood, most reveal themselves to be small and mean compared with the magnificent locations of memory. Frankfurt was different. It seemed that all the world had arrived that year for the Wetterau fair. In one square, the cloth makers’ stalls became a tented city of every colour and weave imaginable: from heavy fustians and gabardines to the lightest Byzantine silks that shimmered like angel wings. From the covered market hall came the warm perfume of unnumbered spices: pepper, sugar, cinnamon, cloves and many more I had never tasted.

I tended our stall in a corner of the market between the paper makers and parchmenters. It was a lonely spot – for all the hundreds of merchants, we were the only booksellers at the fair – and I struggled to make myself heard above the clamour. After so many fearful years of secrecy, I could hardly bring myself to speak.

It should have been Fust. He was our salesman, and he had conceived this plan to show off the first fruits of our labour. But he had cried off the day before, complaining of a fever, so I had come in his place. It was good that I did. I had been too much in the Humbrechthof lately. Fust’s deadline was approach ing and the Bibles were still behind schedule. The constant awareness of it – calculating and recalculating schedules, supplies, man hours – had become a weight that hung heavy on my back. I dreaded the journey. I could not imagine myself away from the project, or it without me. But Fust insisted. ‘Peter will come with you. It will do you good,’ he told me.

After an hour on the road to Frankfurt, I knew he was right. The autumn air pinked my cheeks and cleared my head; the ripe smells of fallen apples and leaves unclogged my senses. Even the clamour of the fore-stallers, who risked the wrath of the authorities by offering goods outside the market, seemed more vibrant than irritant. That night I fell into easy conversation with the other merchants at my inn, staying up far later than I was used to, drinking too much and suffering a sore head next day.

On the first morning of the fair, a total of three men came to my stall. I almost counted a fourth, but he only wanted directions to the tanners. I had little to do but slap at the fleas who had warmed my bed the night before. My pleasure at leaving Mainz grew faint; in my mind I composed long can-tankerous complaints to Fust of how this was a fool’s errand. But that afternoon, the flow of visitors quickened. By next morning, I could barely keep pace with them. Many were priests and friars, but they must have reported what they saw favourably. Soon richer hands were picking up the pages, fat rings brushing the vellum. I saw abbots, arch deacons, knights. And, eventually, an unexpected bishop.

Every half-hour, something like this would happen: I would be standing behind the stall, commending the virtues of my books, when a young man with an ink-stained smock and wild hair would make a commotion, pushing through the crowd until he came to the front. He glanced at the Bible pages, then turned to the crowd and announced loudly, ‘He is a fraud.’

He held the quire open so everyone could see.

‘This man claims his text is perfect, but clearly he has not even read it. There is not a single correction.’