Isaac pulled the square of parchment out of his pocket, and they examined it together. It was many sheets thick, folded in on itself in a complicated pattern. The outside was blank.

‘I’m not sure this is it,’ Isaac said. ‘I’m not even sure you can have a map of a place like the library. It wouldn’t stay put long enough to draw one.’

‘Let me see it,’ Alice said.

Isaac looked down at the map, hesitated for a moment, and handed it over. It looked as though it should unfold, but whenever she tried to tease the layers apart, it resisted stubbornly. She tried to work her little finger between two sheets by wiggling it back and forth.

‘Hey!’ Isaac said. ‘Be careful! You’ll tear it.’

Alice frowned and worked her way around the edge. All at once, she found a place where she could get it apart, peeling two layers away from each other and opening the square like a tiny book. They felt slightly sticky, as though someone had spilled apple juice on it.

Where the parchment had been pressed together, drawn so small she could barely make out the details, was a perfect little map. In the centre of one half of the paper was the anteroom in which they were standing, with its two thick doors. From there a thin green line ran down the length of the page, into the library and snaking through a maze of shelves, crossing the crease where the map had been folded and eventually going off the opposite end of the page.

Isaac looked down at it in the light of his glow-wisp, fascinated.

‘It must be magic,’ Alice said. ‘Do you know how it works?’

‘I have no idea.’

Perversely, that made Alice feel a little better. ‘Mr Black said he got it from something called the Bone Oracle. Have you ever heard of it?’

Isaac shook his head. ‘A friend of mine once told me that you can find practically anything, if you know which book to look in. Some world, somewhere, has what you need, and some Reader probably wrote down how to find it a thousand years ago. There have been Readers a long time, and they may lose things occasionally, but they never throw anything away.’

‘I’ve seen that for myself,’ Alice said, nodding at the door leading into the cavernous library. She traced the green line on the page with her finger. ‘I suppose we’re meant to follow this. But what happens when we get to the end of the page?’

‘I guess we’ll find out,’ Isaac said. He seemed considerably happier now that they were back in the library, and she couldn’t blame him. At least in here he can always hide if someone comes after us. ‘Come on.’