The court documents relating to the Anthropic case revealed the project of ingesting old books was referred to in internal company communications as "Project Panama."

The documents indicated the company's aim was to "destructively scan all the books in the world".

Destructive scanning is the process of shipping books to locations where they can be digitised at an industrial scale.

It includes removing a book's spine so all the pages can be scanned rapidly - and the remains recycled.

"A lot of mystery surrounds Project Panama," Manley, from Barter Books, says.

"The name is new to me, but the reality of the project is not, and has been the subject of much discussion on the bookseller forums."

He does not know that his books are being bought for it or similar projects by other AI firms.

But he says it's also difficult to account for the sales, which appear random with "no rhyme or reason."

They have varied from obscure Latin texts to cowboy novels.

Experts say the diverse subject matter also points to AI, as unusual and rare texts could provide fresh material to improve the training of large language models, the tech which underpins generative AI tools like chatbots.

Professor Emily Hudson, intellectual property specialist at Oxford University, says copyright laws in the UK are different to those in the US.

"The starting point in the UK is that all these acts of copying - creating the training library and doing the training – require the permission of the copyright owner," she says.