If one of your children has their nose in a classic Ladybird book, wrest it out of their hands immediately, wrap it in plastic and stick it in the attic.

On the quiet, and almost unnoticed by the traditional booksellers, the appeal of Ladybird books has grown over the past few years to the point where rare and collectable copies now change hands for around £300. Not bad for a series which, for 30 years, sold for 2s 6d - 121/2p - each.

Often written by teachers, and illustrated until 1980 by well-known children's book artists, they are now benefiting from our increasing love of nostalgia and the comfort of early memories. "They remind me of my childhood - happy times," says collector Karen Strang, who has 2,000 Ladybird books. "I was drawn to their uniformity too; they look great on shelves.

"It's a bug really. Once you start hunting them down, you realise just how many more are out there."

With print runs of thousands of copies, many of the books are still worth only pennies, but there are some surprises.

"The Famous People series published just a few on Indira Gandhi so they're very scarce. I've seen them go for £70 each," says Helen Day, a self-confessed Ladybird addict who runs the www.ladybirdflyawayhome.com fan site.

"I've squirrelled away my most valuable ones because some are worth around £300 to £350 each."

Ms Day, a lecturer, has a 6,000-strong collection of Ladybird books and buys and sells on the internet. "You find them at car boot sales, jumble sales, charity shops and on websites such as eBay and Abebooks.co.uk.

"Specialist bookshops don't know much about Ladybirds and it's possible to get good ones at such cut-price prices."

Opportunities are getting rarer, though, reckons Edinburgh literary researcher Robert Mullin, who runs www. theweeweb.co.uk, a children's book site. Ten years ago, when he started collecting Ladybirds, a whole box of books could be had for as little as £1 at car boot sales or 5p each in charity shops.

No longer. "Thanks to the internet, prices have shot up - particularly over the past five years," he says. "People are starting to see what they can get for their old books and it's becoming harder to find the great bargains.

"Cinderella, a much-loved title, was changing hands five years ago for £5 a copy. Now they're £55 each and if you have one of the really rare copies that had a dust jacket, you can expect to get £250."

The best bargains can still be found in car boot sales, he adds, since people don't realise how much the books might be worth. Charity shops such as Oxfam, which have book experts on hand, do not sell them at knockdown rates.

Most collectors - and Mr Mullin estimates there are hundreds around the country - either try to collect one of everything that the company produced (quite a feat as there were several hundred published in the most popular period, 1940-80) or specialise in a single Ladybird series. These categories are as diverse as Fairy Tales and Rhymes, Animals and Adventures from History.

Prices for Ladybird books seem to depend on two main factors: which series they are from (some are in much greater demand than others) and their rarity. A page on theweeweb shows which series are most popular and which are particularly rare. Some specific books and some whole series are now very scarce and are sought by collectors all over the country.

The Ladybird imprint, now published by Penguin, started in 1940 with Bunnikin's Picnic Party. This was illustrated by Angusine Macgregor, a noted children's book illustrator of the period. Ladybird books went on to cover all kinds of educational and recreational topics, selling in 60 different countries.

They were started by publishers Harry Wills and William Hepworth (Wills & Hepworth) when new technology allowed a single book of a particular size to be produced from a single sheet of paper (naturally enough, the Ladybird book Printing Processes goes through the details).

Each book had a full-colour illustration every time you turned a page, and strongly bound stiff-board covers. For the first 25 years, they also came with a paper dust jacket.

Some collectors hunt high and low for first editions but this can be problematic as many Ladybird books often did not have any details stating whether a book was indeed a first edition or not.

Expect to struggle when searching for particular editions or series. The early, six-book Adventures of Wonk series, for example, is very hard to come by.

The books feature stories about a koala bear illustrated by Joan Kiddell-Monroe. They sell for about £100 per copy with dust jacket, and for between £15 and £60 without it.

The Tinker's Wig series is also a devil to unearth. Published in 1947, it is something of an oddity as it is not only twice the size of a standard Ladybird book but broke with the usual format by printing text on both sides of the pages and using fewer pictures. A copy with dust jacket would sell for £100 to £150; without jacket, you could get £40 to £60.

The rarest Ladybird book - so elusive, it seems, that not one collector has even seen one - is The Computer from the How it works series, produced privately for the Ministry of Defence (MoD) in 1972.

The print run was limited to 100 copies on plain boards without the usual Ladybird copyright information. The simple design and lack of information were at the request of the MoD, since the government body did not want trainee staff to know they were learning from a Ladybird book.

But the fact that the MoD itself deemed the Ladybird approach useful enough for adults is testament to the books' clarity and intelligence. Ten years later, during training for the Falklands War, the British Army used Ladybird's Understanding Maps to instruct soldiers in the art of map reading.

Mr Mullin says it's definitely the nostalgia factor that makes Ladybirds so valuable and could help them to keep growing in popularity. "Once someone's grown up, they want to buy the books they loved for their own children. Then they get hooked and start collecting them for themselves."

There is not yet a society for Ladybird lovers, but a few fans are discussing the possibility of setting one up. Once that happens, copies could be bought and sold even more energetically. Prices, for the next decade or so at least, look like they will continue to rise.

The above article appeared in The Independent on Sunday 26th September 2004.