Decades before eBay and PBS's Antiques Roadshow, Ralph and Terry Kovel were helping the masses sort the treasure from the trash as the leading publishers on antiques and collectibles.

This past year, the couple celebrated the 50th anniversary of the book that started it all, Kovels' Dictionary of Marks—Pottery & Porcelain: 1650 —1850. The book has since been though more than 42 printings, and the Kovels have become an institution in the world of antiques and collectibles publishing.

Working out of their home in Cleveland, Ohio, the husband-and-wife duo have just finished their 90th book. They've also expanded their brand to include a syndicated newspaper column that runs in more than 150 newspapers, a newsletter with a paid circulation of 150,000 and a television show on HGTV.

But books are still at the core of their business. The Kovels' Antiques and Collectibles Price List, now in its 36th edition, is a staple of the market, updated annually with 50,000 different items. The couple's publisher, Random House Reference, declined to reveal sales figures, but the Kovels say the book sells about 200,000 copies a year. Their editor, Dorothy Harris, attributed the Kovels' longevity to their passion for the subject and their work ethic. "They are extremely conscientious and very good at what they do," said Harris. The couple has stayed with the same publisher, or some permutation of it, throughout their entire writing career. Ralph Kovel admitted that he played a little hardball to get their first contract with Crown in 1953. "We wrote them a letter and said, 'Look, if you're not interested, some other people are.' Which, I can say now, at the time was a lie," he said. "We got a return letter with a contract and a check for $500."

The Kovels came to their field inadvertently. As newlyweds, they were furnishing their home on the cheap by shopping flea markets and estate sales. They wanted to be able to interpret the marks on what they were buying, so they began doing research. Half a century later, they're still putting out three books a year with no plans to retire.