Detective fiction is probably the most avidly collected section of the modern second-hand first- edition market. I buy what appeals to me and hope that if I like a book, collectors will too.

In general, value resides only in first editions, and the scarcer they are the better. We wouldn't advise clients to collect with a view purely to investment, though historically prices for rare items have held up very well. When an author becomes famous their print runs rise: Dick Francis's Sixties books are valuable, but after the mid-Seventies he had print runs of 10,000 to 50,000, and the books have little value. In the 19th century first editions were published in minuscule editions: Wilkie Collins's The Woman in White in its original binding would be around £7,000. Later reprints are less expensive: a good-quality early reprint of The Hound of the Baskervilles, for example, would be around £60.

Modern writers are collectible too, though: a first edition of the first Ian Rankin novel is a four-figure book. Buy the paperback to read and the hardback to keep. Look after the book well, and keep the dust wrapper: they can multiply the price by a factor of up to 100.

Agatha Christie remains one of the most collected crime writers. There's a wonderful picture of Christie doing a signing in the Thirties, with a huge pile of her books in dust wrappers sitting beside her. I look at that and wish I had a time machine.

Nigel Williams Rare Books, London, WC2 (020-7836 7757; www.nigelwilliams.com )

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