How high is Mont Blanc? Find out at the Bonhams pen sale on Friday
Here is how to make a fortune from your hobby. Spot an under-appreciated collectable, buy it up, write the definitive guide book about it, then, as prices soar, bung your collection into auction and make enough cash to retire on.

Half a dozen clever collectors have done just that in the past few years. The latest are Andy and Jenny Lambrou: 400 fountain pens from their collection - the biggest ever auctioned - are expected to fetch pounds 50,000 at Bonhams on Friday (11am). The second and final sale on 10 May could bring the total to above pounds 100,000. They published their definitive Fountain Pens Vintage and Modern in 1989, the year after Bonhams launched its fountain pen auctions. The book identified and dated fountain pens, turning them into a mainstream collectable. Their second book, Fountain Pens of the World was published last year.

Fishing tackle and thimbles have also made fortunes for pioneering collector/authors. The finest 400 thimbles from the collection of Edwin Holmes, author of History of Thimbles, totalled pounds 188,575 at Christie's last year. Angling Auctions' first three of a series of six sales of vintage fishing tackle collected by Graham Turner have so far raised pounds 100,000.

The collectable pen market has become more sophisticated. Top-of-the- range standard production pens tipped seven or eight years ago to repeat the success of the Twenties vintage, are today hardly collectable. The Parker Centennial of 1988, for example, sold for about pounds 150 retail but the edition was heavily promoted and not limited in number, so has not kept its value - they go for pounds 80-pounds 120 at auction.

Pen collectors are now being seduced - and here is another familiar collecting trend - with limited editions produced specially for the collectors' market. The German 1994 Mont Blanc Louis XIV, produced in the customary edition of 4,810 (Mont Blanc's height in metres) retailed for pounds 1,100 and immediately attracted a premium of pounds 500, settling to pounds 1,200-pounds 1,600. That is the estimate on the one in the sale.

Best of the rest: Christie's has two high-class remnant sales from interior decorators of yesteryear: the 200-lot Oriental carpet collection of Bernheimer's of Munich, once world-class decorators, Wednesday (2pm), estimates from a few hundred to pounds 450,000: and at South Kensington, Wednesday (10.30am), chic French furniture from the Mayfair apartment of the late Godfrey and Elsa Bonsack. Gold dolphin bath-taps were Godfrey's trademark.