WASHINGTON — “It’s like getting a present,” Christa Cothrel says gleefully. The self-described bibliophile from Arlington, Va., has been getting books in the mail for four years — for free.

Joan Wendland, of Sterling, Va., hasn’t bought a book in 18 months, yet she has shelves full of sci-fi and fantasy, mysteries, nonfiction, audio books and one Jasper Fforde novel autographed by the author.

“He doesn’t come through the States often,” Wendland, 44, says.

Wendland didn’t pay a penny for any of these books. She and Cothrel are two of the thousands of people using the Web site PaperBackSwap to trade books.

The autographed Fforde cost her one “credit,” which Wendland received when she mailed an old book to someone else.

Richard Pickering and Robert Swarthout founded PaperBackSwap in 2004, when Pickering was traveling for work.

“I would buy paperback books at every airport. … I amassed a very large collection,” he says.

After trying to sell them to a used bookstore (“the woman went through five boxes of books and picked out four”) and failing to unload them online, he hit on the idea of a Web site where readers could trade for the cost of postage.

Cothrel, 39, started by listing history books she’d used in college.

“I was amazed at how many people wanted to hear about early Colonial Puritans. Within a day (the books) were all asked for.” She uses her credits to trade for contemporary fiction and fitness books for a class she teaches.

“I don’t know how many books you can write about abs, but apparently quite a few — and think I have all of them,” she says.

There can be a social aspect to book-swapping, too: PaperBackSwap’s organizers try to foster community through online forums. And when Wendland noticed she was mailing a book to someone locally, “I called her and said, ‘Instead of me paying postage, why don’t I meet you for lunch?’ ”

PaperBackSwap isn’t the only such site. Here are details on some book-swapping sites:

BookCrossing.com: Has more than 730,000 registered users and about 5.3 million registered books. It’s free, but it’s not exactly a swap site. Users “release” books into the “wild” in public places (parks, coffee shops). Those who find the books register their finds on the site, tracking their progress around the world.

Bookins.com: Has more than 30,000 users and more than 512,000 books available. The receiver pays $4.49 to cover postage; the sender pays nothing. The site, which also handles DVDs, coordinates transactions, so users don’t deal with each other directly.

BookMooch.com: Has 25,000 active users and about 52,000 books. Sender pays postage.

NovelAction.com: About 7,000 books are available. The company doesn’t disclose the number of users. It costs $25 per year plus postage. Users can pick out an entire box of books at a time, and then ship back an equal number of quality books.

PaperBackSwap.com: The site has “tens of thousands” of users, according to co-founder Richard Pickering, and more than 2.7 million books available. The sender pays postage. It just deals in books, but points earned by sending books also can be traded on sister sites SwapaCD and SwapaDVD.