Image copyright Phil Kirk/Oxfam

The rise of online has helped revive the second-hand book market, but what impact has it had on traditional, second-hand book shops?

When a box of old books arrives at Oxfam's Witney bookshop in Oxfordshire, it's a bit like opening a treasure chest.

Manager Sally Lee and her team pile up the bulk of them to go straight on the shelves: the Jilly Coopers, the Lee Childs and the John Le Carrés. Some, the ones that have been dropped in the bath or scribbled in, will have to be sent for recycling.

And they always keep a sharp eye out for anything that might be a signed copy or a first edition.

But there's a whole lot of other books to look out for these days that can be surprisingly valuable.

"In my time the retail of second-hand and vintage books has changed beyond recognition," says Ms Lee, who has been working for Oxfam for two decades.

"Things like Ladybird books, lovely old-fashioned or collectible children's books - in the past, people weren't really interested. If they remembered books from their childhood, the chances of finding it in a bookshop near them would have been nil."

But thanks to the arrival of online second-hand book retail, there is now a whole new market for what Ms Lee calls "cheap collectibles" - mid-market books worth more than a couple of pounds, but not worth sending to auction. And Oxfam is making the most of it.

Image copyright James Beck Image caption You might just find something unique on the second-hand book market

Today she has been listing online some magazines from the 1940s and 50s about the Royal Family for £50 that she is confident will find a buyer.

Rare old books, such as Adam Smith's own copy of The Wealth of Nations that has just sold for £908,000 and first-edition Harry Potters, have always sold for eye-watering amounts.

But now there is also money to be made from the kind of books that used to gather dust in the corner of charity shops: the thrillers, romances, celebrity biographies, and dog-eared children's books.

There are no official statistics for the size of the second-hand book market. But a survey by Statista found that in both the UK and the US more than half of us are choosing to buy more books second-hand than we buy new.

And Patrik Oqvist from World of Books, the UK's largest second-hand book retailer, estimates the market is growing by 8-10% a year.

"There's no stigma to buying second-hand now," says Mr Oqvist. "We take them to the beach and spill coffee on them, but they don't stop working because of that." Then there's the lower price, and the fact that you're recycling.

But there's also the chance of finding something unique. World of Books had a call last year from a grandmother in Australia who had ordered an annual that she remembered owning as a child, full of quizzes, mazes and puzzles. When it arrived, she found it was her own original copy, complete with the inscription from her parents to her.

Image copyright World of Books Image caption Online sellers like World of Books store used books in vast warehouses

World of Books started when its founders noticed surplus books being chucked out of a charity shop, destined for landfill. They bought them on the spot, determined to give them a second life.

They've just opened a huge new warehouse in Coventry that will allow them to store more than the 2.8 million books they currently have in stock.

Smaller rival, Lancashire-based WeBuyBooks, is also expanding, opening a second warehouse in Rossendale.

"The majority of books we buy and sell are the everyday books people have on their shelves," says Ben Wadsworth, the firm's marketing manager. "Things like textbooks, anything academic tends to hold its value."

Online giant Amazon spotted the potential of the second-hand market, and in 2008 bought AbeBooks, a huge Canadian marketplace for used books. Like Ebay and Amazon itself, AbeBooks simply matches buyers and sellers without handling any of the books themselves, and acting as another platform for firms like WeBuyBooks and World of Books.

"It's all about volume," says Richard Davies from AbeBooks. Sellers can scrape a profit from prices as low as a penny plus postage and packaging, he says.

Image copyright World of Books Image caption World of Books' new Coventry warehouse is the size of four football pitches

But what about the traditional, second-hand book shops? Aren't they undermined by the online trade?

Mr Davies says not. "Those shops are also serving a global community of book lovers now," he says.

He argues the online trade actually helps second-hand shops. "When they close their doors they're still selling. At the weekend if they're closed, we're promoting books on their behalf, and on Monday hopefully they've got some orders to process."

Pom Harrington, who runs an antiquarian bookshop in London, agrees. He's noticing a growing market in Hong Kong for European first editions of Karl Marx's works that in pre-internet days just wouldn't have found him.

And he says online has had an "immense" impact on the used-book sector, by putting information about the value of unusual books at everyone's fingertips, helping those less in the know make the most of valuable finds.

Image copyright WeBuyBooks Image caption WeBuyBooks' Ben Wadsworth says they make their money out of "everyday" books

In the past charity shops were the only destination for piles of unwanted books. Now firms like WeBuyBooks and World of Books are offering book owners the chance to sell their used books to them, using specially designed apps.

But will this hurt charities? World of Books' Patrik Oqvist argues its app will attract additional books onto the second-hand market rather than cannibalising donations to charity. World of Books is also about to add an option to its app, Ziffit, which allows book owners to donate the proceeds from their used book to charity.

Oxfam says the rise of online, by making the market more transparent, and providing its volunteers with expert guidance, has helped to boost its income from books.

But that doesn't mean you can make money out of every volume. There is still such a thing as an unwanted book.

Image copyright Phil Kirk/Oxfam Image caption Oxfam says the internet has helped its book sales

Charity shops and second-hand shops don't have space for several repeat volumes of once-popular titles. And even the online warehouse-based operations have some books they don't think it's worth stocking.

The Dan Brown series, for instance, sold in such great numbers originally they simply would never all find buyers so only the ones in tip-top condition are kept for resale.

"The perfect example is Fifty Shades [of Grey]," says Ben Wadsworth at WeBuyBooks. "We were reselling it at first, but after the hype had died down and everyone had read it, there were hundreds of thousands of copies for sale on the internet, but no demand."

Books like that, victims of their own success, are still destined for pulping and recycling into cardboard, which can at least be used to package up another second-hand book.