The £7.5m book at centre of Small Prophets plot
BBCLibrary assistant Eleanor de Spretter Yates was watching Mackenzie Crook's BBC comedy Small Prophets when suddenly hear ears pricked up.
It was the mention, in episode four, of "a book, a bird book" which piqued her interest.
And as she watched, she was ecstatic to find that the very book that featured in the BBC Two supernatural comedy was the one she sees every day at Liverpool Central Library.
She did not need to wait to find out why two suspicious characters should be on the hunt for it, because she already knew what was so special about this particular tome – Birds of America.
It is one of the most expensive books in the world.
Birds of America, published in the mid-19th Century by ornithologist artist John James Audubon, is one of the library's more valuable treasures.
It contains life-size, meticulously hand-coloured images of 435 birds from the United States produced from the 1820s onwards, each taking about 60 hours to complete.
People had never seen anything like it before, de Spretter Yates says.
"When these came out, some of the scenes of the birds in flight, birds in motion, but also birds eating their prey… the violence of it, these were quite sensational images when they were first released," she says.
While artefacts such as the city's Charter from 1207 are priceless, Birds of America is not far behind. The library's copy – one of very few in the world on public display – is insured for £7.5m.
To put that into context, if one were to borrow another book and fail to return it, it would take over 200,000 years to rack up the equivalent in library fines.
Given its value, the library has had to go to great lengths over the years to protect it. And some staff have even had to risk their lives for it.
During the Liverpool blitz between 1940 and 1942, the library was struck by a bomb. As fire raged and firemen tried to douse the flames, a librarian went back in, heading for the flooded basement to retrieve valuable artefacts, including Birds of America.
It would have been no mean feat, given the massive tome is 26in (66cm) by 50in (127cm), or "double-elephant folio".
To this day, there are watermarks on some of the pages from the fire hoses.
For three years, when Liverpool Central Library was being redeveloped in the early 2010s, there was the question of what to do with this treasured artefact. It could not be just kept in a cardboard box in a storeroom.
So, along with some of the library's other extremely valuable collections – letters from Admiral Horatio Nelson and drawings by 19th century post and illustrator Edward Lear among them – it was kept in temperature-controlled storage 500 ft (152 metres) below ground in a salt mine in Winsford, Cheshire.
The mine was rumoured to have been home, briefly, to the Crown Jewels during World War Two.
The book's pages are delicately turned regularly to ensure they do not fade in the light, and kept behind glass in a temperature-controlled box.
With only 120 complete volumes known to have existed, it is rare for the public to be able to be able to glimpse Birds of America.
So, with public sector finances tight these days, has the council ever been tempted to sell it?
Considering that the most expensive sale of an edition at auction, in 2019, saw it make about £7.25m, Liverpool Central Library's Birds of America – insured for £250,000 more than that - could be the most valuable in the world.
"Audubon illustrated five species that are now extinct, three species that are thought to be extinct, and five mystery species – we don't actually know what they are," says de Spretter Young. "It's that level of knowledge that outweighs any financial gain."
Given her enthusiasm for the book, it is no wonder that de Spretter Young was so thrilled to find it was to become central to the plot of Small Prophets.
"As soon as they said 'there's this big expensive book' I was like, 'Oh, I wonder …'" she says.
"And when they said [Birds of America], I did a little whoop!"
Birds of America can be seen in the Oak Room of Liverpool Central Library from Monday to Saturday. Small Prophets continues on BBC Two, and the whole series is available on iPlayer.
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