But the floor was shuddering violently underneath them. It felt like a passing underground train, but more worryingly close to the surface. Then the floorboards directly ahead buckled upwards in slow motion, and something clawed and dark tore its way up and through. It dragged itself up into the passageway in a vast clashing of gears and clinking of metal. It was all oil-smeared steel except for the head, which was glass-panelled on either side to make two huge flat translucent eyes. It was clearly from the same root design as the metal creature that Kai and Vale had fought two nights ago, but smaller and faster.

‘What’s this?’ Bradamant asked calmly, her words oddly distinct against the sound of splintering wood, grinding metal and distant howling.

‘I think it must be the Iron Brotherhood,’ Irene answered. ‘They probably followed Silver.’

‘Oh, this is simply getting ridiculous,’ Bradamant sniffed. ‘Which way next?’

The insectoid robot head swivelled to focus on Irene and Bradamant. It took a jointed pace down the corridor towards them, the claws attached to each segment of the body dragging it along and leaving horrible gashes in the wood. Its top scraped the ceiling, bringing down cobwebs that had probably been centuries in the making, leaving a long swathe of scoured white plaster in its wake.

‘Go right,’ Irene shouted to Bradamant on no particular evidence, and ran in that direction. She was already calling vocabulary to her mind – words for gears, joints, pedals, steel, glass, struts and nuts and bolts. But there was always the chance that the construct would decide to chase Silver and the werewolves rather than them, and it seemed a shame to wreck it if so.

‘It won’t work, you know,’ Bradamant said, catching up and outpacing her. ‘Do you seriously think that thing won’t chase us?’

‘It’s worth a try,’ Irene gasped. She turned and looked back over her shoulder.

The iron automaton came jolting forward in a screeching rattle of steps, then halted as it reached the junction. With a whirr the head turned to edge itself into the passage that Bradamant and Irene were running down. Its shoulders began to creak after it, manoeuvring so it could bear down the passage after them like an oncoming train.

Irene and Bradamant looked at each other.

‘I’ll do the gears if you do the joints,’ Irene said.

‘Right,’ Bradamant said. ‘Give it a moment so that it can block the junction.’

The robot managed to half-negotiate the turn. Its claws dug into the floor as inner springs rewound themselves. The huge lenses set into the head reflected the two women, mirror-like. If they were in fact windows, it was impossible to see who might be lurking behind them.

‘Gears, lock up!’ Irene shouted, pitching her voice to carry as far as possible. ‘In head, in claws, in body, and in every part which can hear me – gears, seize solid and stand firm!’

The robot came to a standstill in a horrific mechanical screaming of blocked joints and gears. Even the distant howling of the werewolves was drowned out. Wires and cables tensed and broke. One claw rotated backwards, caught itself in the floor at an angle, and snapped. And a fragment of steel went flying, pinging off the wall with a high-toned ring of metal, audible even over the noise of the machine destroying itself.

Both women turned, and ran down the corridor away from the thing, past closed offices and storerooms. The air was full of fresh dust, the smell of oil and burnt metal. A part of Irene’s mind wondered if it’d make tomorrow’s front pages. Probably. She didn’t like making headlines. A good Librarian was supposed to read headlines, not make them.

‘There!’ Bradamant pointed unnecessarily to a stairway ahead of them. They plunged down it at a run, Bradamant swinging wide on the banister at the curve and almost hip-checking Irene. The door at its base opened on to the ground floor, revealing a room full of shells and corals. Several family groups turned to look at them disapprovingly.

Irene smiled her iciest smile, brushed some of the dust off her skirts, and took a firmer grip on the precious ledger. Behind her, Bradamant whispered something to the door lock. Irene couldn’t quite make it out, but it had the cadence of the Language.

Hopefully they had a couple of minutes before any werewolves, Fae, Iron Brotherhood, or other book-hunters caught up with them. Irene spotted a small office on the other side of the room and caught Bradamant’s eye. ‘Over there,’ she suggested, jerking her chin towards it.

‘Absolutely,’ Bradamant agreed.

The two of them walked decorously across the room, skirting glass cases full of dried sea anemones, brittle polyps, and other brightly coloured objects that were probably happier when they’d been underwater. With a polite nod to an elderly man shuffling along behind a walking frame, Irene quietly tried the handle of the office door.

‘Is it shut, dear?’ Bradamant enquired quietly.

‘Oh no,’ Irene said, keeping her voice down. ‘In fact, this door is open.’ The Language rolled in her mouth, and the handle loosened under her hand, turning obediently to let the pair of them in.

‘Not bad,’ Bradamant said, closing the door behind them. She looked around for a key, saw none, and muttered, ‘Door lock, shut.’ The lock clicked to again.

Irene glanced round the room. It was clearly someone important’s office: the desk and chairs were newer than the ones downstairs, the pieces of artwork and diagrams hanging on the walls had frames, and there wasn’t any dust. ‘We’d better not take too long,’ she said, walking over to the desk. She sat down and flipped the ledger open. ‘Someone might come in at any moment.’