I probably never would have set foot in the Georgetown Book Shop in Bethesda if I hadn't wanted to talk to owner Andy Moursund about the business of used-book stores. But meeting Moursund and witnessing his passion for acquiring not only books but knowledge makes me think I may just wander into more used-book stores in the future.

It's not that I don't go to bookstores, because I do, and I love them. I like to hang out at Borders, browsing, taking my kids, looking for presents and, of course, buying books.

Used-book stores are something else. They seem impenetrable and intimidating to me. But an e-mail from Moursund a few months ago telling me about his world of books made me wonder about the differences between the two types of retailers. I was curious, too, whether most customers are like me: Do new- and used-book stores attract the same customers?

It turns out that used-book stores operate in their own little universe, more or less outside the world of new-book stores. Shopper crossover between the two is minimal, and the motivations of shoppers are often quite different.

It also happens that used-book stores are getting eaten alive by the Internet. It's a shame, because we need more people like Andy Moursund in our lives.

The most obvious difference between used-book stores and regular book shops, of course, is price. Beyond that, the used ones differ in that they often specialize in particular subjects, depending on an owner's passions. Moursund has military history as one emphasis, for example, while other shops can be found around Washington that specialize in psychology, maritime history, Americana, politics, science fiction and art.

"I'd rather be really good in four or five subjects than mediocre in a thousand subjects," Moursund says.

There are plenty of inspired, knowledgeable people working in regular bookstores, of course. But the owner of a used-book store tends to view his or her shop as a knowledge center, not just a business. Moursund wants nothing more than to share the facts and ideas in his books, and his own head.

"You have to see a social perspective to it beyond making a living," Moursund said. "Otherwise, it would get boring."

This kind of attitude is typical of used-book dealers, said Barbara Meade, co-founder and co-owner of Politics & Prose, a vibrant independent bookstore in Washington.

They "believe that time has taken care of . . . weeding out all the ephemeral books, so what they're dealing with really is true gold," Meade said. And that "what we're dealing with is a lot of fool's gold."

And Meade doesn't necessarily dispute that assertion. A lot of what general bookstores must sell, in fact, is not that great, she admits.

Knowing the used-book business, or at least the subjects being sold, is critical because one of most difficult aspects of running a used-book establishment is finding the books to sell. Often it's a function of visiting the homes of people with large libraries to get rid of, which means the shop owner must be able to judge a variety of collections quickly. Moursund has bought groups of several thousand books primarily to get a few hundred of the best titles that he spotted amid many more uninteresting and not-so-valuable volumes. Sometimes dealers will cooperate on large collections, with one taking the books on philosophy and another the books on history.

Much of Moursund's stock comes from his own customers, some of whom have frequented his store for 20 years through three locations. These shoppers are driven by a desire to collect knowledge, if not read it.

Julie Marquette, owner of Bonifant Books, a general-interest used-book store in Wheaton, agrees. "I have customers that come in that might buy half a dozen books a week, and I know they go to other stores. They can't be reading them all. It's that collecting, that hoarding, of information."

It's largely a male characteristic, Marquette says, noting that 80 percent of her customers are men. Women are more likely to read fiction, to share their books with others and to go to the library, she says, while men are more inclined to want great quantities of books of their own, mostly nonfiction. Even if they can't read all that they buy, men like accumulating accounts of all of Napoleon's campaigns, for instance, or the full blue-water-sailing oeuvre.

But these shoppers are not as reliable as they once were. Customers are being lured away from the neighborhood shops by the promise of finding the titles or information they want online. Used-book Web sites list books from dealers worldwide, making the search for an out-of-print book as easy as typing its name into a search engine.

Traditional new-book stores have insulated themselves from this fearsome competition by becoming places to browse or just hang out with a latte. But used-book stores aren't about such frills.

At Bonifant, business is off 20 percent in the past two years. "This isn't Borders -- it doesn't take that much," Marquette said. "You lose a few customers each day, and it adds up." She figures she'll be out of business in a few years.

Moursund has responded by adding historical posters to his collections of history, art, photography, baseball, cooking and children's books. "You just can't make it on used books alone. You just can't," he said.

He prints the posters himself from his own collections of old book jackets, political pamphlets and magazine covers, all with striking graphics. He sells many online, but hanging in the window of the store they are helpfully eye-catching. The posters offer a raw, unfiltered look at history, whether it's the patriotism of World War II America, the rise of communism or racism in the 1950s.

And when someone walks in the door enticed by a poster, Moursund is satisfied that he is spreading knowledge, even if that customer isn't reading a book.

"When someone in their twenties starts looking at posters," he said, "what a new world can open up to them."

Or to anyone.

If you have a question, comment or concern about what you see when you shop, send an e-mail to sellingus@washpost.com.