And then, as suddenly as it started, the mermonkey judders to a halt. Its right hand draws back from the keyboard, and the left hand holds the hat out once again. Its eyes wink out, and silence returns to the shop, leaving only a hint of acrid smoke in the air to suggest that anything happened at all. Then there’s a ping! from the typewriter, and a card is ejected from it, fluttering down to the floor at Violet’s feet. She picks it up.

“And that, Violet Parma, is your prescription,” says Jenny Hanniver. “It’s the book the mermonkey has chosen for you. Not, perhaps, the book you want, or the book you were expecting. But, very possibly, the book you need.”

Violet rejoins us. The card in her hand is just like the one around her neck – a drawing of the mermonkey on one side, and a line of letters and numbers on the other:

4 - 2 - E - Pu - 78

“But what does that mean?” she says. “Is it code?”

“Herbie?” Jenny looks at me with a twinkle in her eye. “Can you remember how to read the mermonkey?”

I scratch my head under my cap. “Um, I think so. The first number is the floor we should go to?”

Mrs Hanniver smiles, and indicates the stairs, faintly visible deep inside the shop. “Call me if you need help,” she says, and turns away to a pile of books waiting to be sorted on a nearby table.

“Come on,” I say to Violet. “This way!”

“And the rest of it?” says Violet as we reach the fourth floor. We’re alone up here, with no sound but the creaking of old wood and the wind at the windows. We can see three rooms ahead of us, one after the other, all of them as stuffed to their high ceilings with books as the rest of the place.

“The ‘2’ means the second room,” I say, leading the way.

“And the ‘E’?” says Violet, when we get there. “Is it … East?”

“It is,” I say. “And it’s easy to orientate in this town – the sea is always the same direction. So this is the east wall. What was the next part of the code?”

“It just says ‘Pu’.”

“‘Pu’ is for purple,” I explain. “See, all the shelves are different colours. Purple’s the one-less-than-top shelf.”

I grab a library ladder and hook it to the rail on the ceiling.

“There you go, Violet. It’s your book, you can have all the fun.”

Violet climbs. “The last part of the code is 78,” she calls down.

“That’s the book itself,” I call back up. “Just count them from left to right. If you get to the end of the shelf before 78, then count back…”

“OK,” says Violet, counting under her breath and tapping each book spine with her fingertip. And then: “I think I’ve got it!”