‘So I should . . . what? Just grab a book at random and flip through it?’ Alice had a sudden vision of spending the rest of her life in here, paging patiently through ancient folios while she grew old and grey and wrinkled and the library dust settled on her like a cloak.

‘We’re not so primitive as that,’ Mr Wurms said, with a smile that suggested he understood her private horror. ‘Let me introduce you to your new friend.’

He bent and fumbled under the table, then came up with an open-topped wooden crate half filled with straw. Nestled amongst the brittle stalks Alice could see something black and glistening. Mr Wurms reached in and lifted the thing out onto the tabletop with a squelch.

Alice had to fight two very strong instincts. The first was to run; the second was to find the heaviest book on the shelves and bash the horrid thing until it was a puddle of goo. It looked vaguely like a leech, but the largest leech she had ever seen, black with grey stripes and the size of a small dog. One end, which she assumed was the head, was equipped with a round, puckered sphincter of a mouth and two parallel ridges, along which were dozens of hard black dots that might be eyes. Rousted from its box, it wriggled helplessly for a moment and then rose slightly on its rippling flanks.

Vomiting also presented itself as an option to Alice, and she fought down bile at the back of her throat. She tried to tell herself it was no more awful than an earthworm, even though it was big enough to stretch its gummy mouth around her fist.

‘What is that?’ She was unable to keep a faint quiver out of her voice.

‘It hasn’t got a proper name, I’m afraid,’ Mr Wurms said, looking at the creature affectionately. ‘I call them “seekers”, after their function. They have the nearly unique talent of being able to find a book with a scrap of magic in it even from a distance. I suspect they can smell it, somehow, or perhaps they have some other sense that we lack. You can use it like a bloodhound.’

‘Does it respond to commands, then?’ said Alice, feeling a little hysterical. ‘“Stay” and “heel” and so on?’

‘No,’ Mr Wurms said, ‘I don’t believe it has ears.’

He took a leather cord from the box and threaded it into a metal eyelet screwed directly into the leech-thing’s back. Mr Wurms knotted the cord neatly and presented Alice with the other end.

‘Just give it a tug, and it will get the message. When it starts trying to climb the shelves, you’re getting near the book you want. Then just show it the nearby volumes until it likes one of them, and flip through that one until you find the scrap. You’ll know it when you see it, I assure you.’