A Rare Book of Cunning Device is another little Rivers of London diversion, and this one has the added distinction of being an Audible exclusive. At around half an hour, it is very much a snack rather than a full meal, but it is a rather tasty snack, particularly if you happen to enjoy libraries, supernatural disturbances and Toby the dog.

The story begins when Peter is called to the British Library to investigate what appears to be a poltergeist. Books are being moved, objects are turning up where they should not be and the librarians are understandably unimpressed by the possibility of something supernatural rummaging around amongst the nation’s precious collections.

Personally, I think librarians dealing with a poltergeist is an excellent premise.

Peter is accompanied by Professor Postmartin and Toby, which immediately makes this feel like a proper little Rivers of London adventure rather than a disposable extra. Both are fan favourites for good reason, and having them involved gives the story plenty of personality. Postmartin brings his usual mixture of enthusiasm, knowledge and slightly suspicious behaviour, while Toby is, well, Toby.

The investigation takes Peter down into the less glamorous depths of the British Library, where the mystery becomes rather more interesting than a simple ghost story. What initially looks like a poltergeist turns out to involve an ancient magical device, something that is not quite a book despite being found amongst the books, and which seems to have a mind, or at least an awareness, of its own.

This is where the story really hits the sweet spot for me.

Aaronovitch takes something as wonderfully ordinary as a library and introduces something completely bonkers into it. The British Library is already a place filled with rare, obscure and occasionally rather peculiar objects, so discovering that one of them might be magical feels entirely appropriate.

There is also something amusing about the practical problem Peter faces. It is one thing to discover a supernatural creature lurking in the dark. It is another to discover a magical object loose amongst an enormous collection of irreplaceable books and realise that somebody has to work out what it is before the librarians become even more unhappy.

And you really do not want to annoy librarians.

The story is short, so there is not room for an enormous investigation or a complicated mythology surrounding the device. Instead, it feels like a glimpse into another ordinary day at the Folly, where Peter can be called away from whatever he should be doing to investigate something deeply peculiar happening somewhere in London.

That smaller scale is part of its charm.

The humour is also exactly what I have come to expect from the series. Peter’s dry observations, Postmartin’s eccentricities and the sheer ridiculousness of the situation all combine nicely. There is something particularly entertaining about treating a magical object causing chaos in the British Library with much the same professional irritation as any other workplace problem.

And then there is Kobna Holdbrook-Smith.

He once again deserves full credit for his narration. His Peter Grant is so well established by this point that hearing him immediately puts me back into the Rivers of London world. He handles Peter’s conversational delivery brilliantly, while giving Postmartin and the other characters enough individuality to make the dialogue easy to follow.

He also understands exactly where the humour lies. The comedy is rarely pushed, and his timing allows the jokes to land naturally. Even Toby seems to acquire a personality simply through the way Holdbrook-Smith handles the scenes around him.

If I have one complaint, it is the obvious one. I wanted more. The magical device is such an intriguing idea that I would happily have spent considerably longer investigating what it was, where it came from and what else it might be capable of.

Still, that is a minor complaint.

A Rare Book of Cunning Device is a short, funny and thoroughly enjoyable Audible exclusive. It gives Peter a wonderfully peculiar case, puts Toby and Postmartin back in the spotlight and turns the British Library into another entertaining corner of the supernatural London Aaronovitch has created.

It may only take half an hour, but with a poltergeist that isn’t quite a poltergeist, an ancient magical device and one very good dog, there is plenty here to enjoy.

And perhaps the real lesson is simple.

Never underestimate what might be hiding in a library.

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