Your bookshelves may hold hidden treasure

If your bookshelves bulge with your late great aunt's favourite reading, then they may also be lined with gold. Second-hand books can be surprisingly valuable. Although the principal rule of the second-hand, or antiquarian, trade is: "A rare book is one you don't have", you might still find the odd nugget, particularly if great aunt Maud had a taste for scouring second-hand bookshops, or house clearances.

In ascending order of importance, age, absolute rarity, beauty, and the importance of the text have the greatest bearing on a book's value. Associations can also be important. A selection of bible readings, published in 1850 and which was worth about 50p in its own right, was recently sold for pounds 1,000, because it was inscribed to a sick soldier in the Crimea, and signed "F. Nightingale".

Six late-18th century Gothic novels, listed as being "really horrid" by a character in Northanger Abbey, are of little literary merit. However, Jane Austen fans are prepared to pay an arm and a leg for any surviving copies.

A book will not be valuable simply because it is old and an important literary work. Even an old book may be one of many copies around today. As a general rule, however, any pre-1800 text, provided that it is rare, will be worth something.

Even recent works can be sought-after. For example, a presentation copy of The Satanic Verses should fetch considerably more than its cover price, as the text has become significant, and its author became disinclined to court publicity. In contrast, it is said to be difficult to find a copy of Baroness Thatcher's memoirs that is not signed.

Some of the very earliest Penguins are worth a few pounds. For example, the first Penguin, Maurois' biography of Shelley, is worth pounds 80 to pounds 100 in decent condition. A first edition of Naked Lunch will also be in paperback, and is worth around pounds 200.

Several 20th century poets published their first volumes privately, which they often later disowned. These publications would be in paperback, and there is a market for these usually slim volumes. A copy of W H Auden's first book of poetry was published by Stephen Spender. The book, bound in coloured paper, would fetch several hundred pounds.

A few coffee spots, or one missing page can cut a book's value dramatically. At the other end of the scale, 20th century texts can be worth four figures by dint of a dust jacket, especially if it is a popular text.

For example, a first edition of Ian Fleming's first Bond novel, Casino Royale, in its dust jacket could fetch about pounds 2,000 in good condition. An immaculate copy recently sold for $12,500 (pounds 7,900). However, if the dust jacket is shredded or missing, the value falls to about pounds 100. Similarly, a first edition of The Great Gatsby, which is renowned for its dust jacket, will sell for $15,000 to $25,000, depending on condition. Without the dust jacket, it will be worth a 20th of this sum.

If you are interested in collecting for investment, Arthur Freeman of leading antiquarian bookseller Bernard Quaritch, which deals mainly in pre-20th century texts, recommends developing an area of expertise that is, as yet, relatively unexplored, and in which you may get a step ahead of dealers. "I would be surprised if anybody had yet fully researched, for example, the history of accountancy textbooks, and there are a lot of them around. The catch is - how much would anyone pay for an accountancy textbook?"

To develop your knowledge of the field you should go to antiquarian books fairs, such as the Provincial Book Fair, which travels around the country, and is held at the Russell Hotel, London, once a month.

You should also browse around the bookshops in Cecil Court WC1, which deal in 20th century texts. It is also a good idea to read publications such as The Book Collector, available to order from 0181 200 5004.

Don't try to get valuations over the phone, it is impossible to value a book without examining it. And don't expect a dealer to take the time to make a free valuation, unless you are a prospective customer.