Children's books: Have you got a fortune sitting on your shelf?

If they're rare and in good condition, children's books – old and modern – can be valuable

The dust has now settled on the release of the last Harry Potter book and as a "muggle" – for the uninitiated, that's an ordinary, non-magical person – you're almost certainly aware of the incredible prices fetched by first editions of J K Rowling's books.

Yet the idea of big prices for children's books is nothing new, as it's a market that has been flourishing for years, and is one which all the specialists agree will continue to do so.

While there's no disputing the quality and popularity of the Harry Potter books, they have definitely benefited from a lot of hype at their launch – with everyone wanting a little slice of the J K Rowling magic.

However, Dr Philip Errington, children's books expert from Sotheby's, urges people to take note of what the auction house is looking for in a first-edition Harry Potter.

"The only value is attached to the first three books, and in particular, to the first: The Philosopher's Stone. We are looking for the original trade hardback of the book, published by Bloomsbury, with the numbers 10987654321 on the publisher's imprint page," he says.

This book only saw 500 copies published, because Bloomsbury, as is the practice with many new authors, only printed a small number to test the market.

In May of this year, Bloomsbury Auctions (no connection to the publisher), sold a signed and inscribed first edition of The Philosopher's Stone for an auction record price of £27,370; previously, the auction house had sold one unsigned in 2002 for £19,450.

As the condition of each book is unique, an auction house can only offer a guide price, and must then wait to see what people will offer for it. When very good or "mint" condition copies appear on the market, prices can shift dramatically.

This was no more evident than with Wind in the Willows, first published in 1908. Few come complete with their dust jackets, so when one did appear at auction in 1998, it made £39,000 at Sotheby's – proof of how critical the condition is in determining value.

The bulk of the market is dedicated to classics such asWind in the Willows and Alice in Wonderland, along with Tolkien's Hobbit tales, and works by Beatrix Potter and C S Lewis.

"Books by the likes of Roald Dahl from the 1960s, such as Charlie and the Chocolate Factory, are very collectable," says Dr Errington. "But prices are quite sensible compared to what people pay for J K Rowling."

There's also a big collecting patch from the 1920s and 1930s, where you find books such as Swallows and Amazons, Peter and Wendy (better known as Peter Pan), E Nesbit's The Railway Children from 1906, and Winnie the Pooh.

Books from the late-18th century are highly sought-after, and rare, particularly if they are in good condition.

Glance at any book collector's list, and you can almost always find a film adaptation; movies undoubtedly help book prices.

This was evident with Tolkien's Lord of the Ringss. Tolkien's books rose significantly in price when the films came out, but have now settled back down again, albeit at a slightly higher level than before the movies were produced. This is a critical factor when buying for investment: never buy when the hype is at a high, because invariably the prices are too high.

"I think there was 'ultra hype' for modern authors contemporary to J K Rowling," says Bloomsbury Auctions' Roddy Newlands. "But now people have discovered that they aren't as scarce as they thought. Philip Pullman's popularity kicked in with Northern Lights, but he had already been writing for some time before that." He adds that while copies of Northern Lights used to sell for about £6,000 at auction, their value is now about £1,500 to £2,000.

That said, the market as a whole is a healthy one. "The market is extremely buoyant," says Dr Errington. "We have English literature and children's book sales twice a year, and they are one of the highlights of our book calendar."

His advice for those looking for the next "big" modern children's author?

"Look to see who's in vogue at the moment, and who is up and coming; you may then find that the first books of that author at some point appear at auction."