Part of the appeal of collecting autographs–and one of the challenges–is that for virtually any historic figure or area that piques one’s interest, whether art, music, politics or science, there are likely to be signed documents that can be ferreted out and collected. "It is almost limitless. Whatever you love, there are manuscripts out there," Raab says.

THIS BROADSIDE of the Emancipation Proclamation was signed by President Abraham Lincoln and Secretary of State William Seward in Philadelphia in 1864. The estimated value is $650,000 to $850,000. (Photograph courtesy of Christie’s Images Ltd.)

One can amass a wide-ranging collection like that of David Karpeles, a mathematician turned residential real estate investor based in Santa Barbara, Calif. Karpeles got the bug for autographs in the late 1970s before they were recognized as having value as investments, and in less than a decade he amassed 1 million items, many of which rotate through the nine museums he has funded across the United States.

Among Karpeles’ prizes is a copy of the Emancipation Proclamation signed by Lincoln. He bought it at auction in 1978 for $40,000. "I thought it was crazy. Something like that should be worth millions," he says. While it may not be worth millions yet, a similar document signed by Lincoln went for $688,000 at auction at Christie’s last November.

Karpeles is an unusual case. For most collectors, dealers advise focusing on one or two topics and on buying the best. "The more important it is, the better it is and the more chance you have of appreciation," Raab says.

The autograph market is not immune to hucksters; experts advise collectors to tread carefully. First, they should know exactly with whom they are dealing. Dealers often develop relationships with serious collectors and can serve as their representatives at auctions. The Professional Autograph Dealers Association has stringent membership requirements; its roughly 50 members guarantee the authenticity of the work they sell.

Opinions differ on the best source for high-quality autographs. In addition to dealers, large auction houses hold autograph sales regularly, and dealers like Raab and Lowenherz recently began selling at high-end art fairs, including the annual Connoisseur Fair in Palm Beach. Unfortunately, a few apocryphal stories aside, opportunities for collectors to unearth treasures at flea markets or yard sales are becoming ever rarer.

Horowitz, the Washington attorney, clearly enjoys seeing his treasures hanging on the wall. The Whitman note was a wedding gift from his wife 31 years ago. "We both love it," he says. "When I see the Whitman, I think, ‘Oh God, I married the right lady.’"

Louise Kramer is a freelance writer based in New York.