Two miles from the world’s largest library stands a tent reinforced with bamboo and duct tape. A single bulb — dark until the generator comes on at night — hangs from the center. On the metal lampshade someone has scrawled: “Let there be knowledge.”

The Occupy D.C. Library in McPherson Square is just down the path from the Temple of Clothing and the Information Tent. It started out as a box of books. Then the rain came.

Eric Gustavson erected a tent to keep the growing collection of donated books dry. The 50-year-old construction worker hasn’t found much work since the real estate market collapsed. Now he oversees a cramped puzzle of shelves, tables and plastic crates marked “Hot Issues,” “Philosophy,” “Spiritual” and “International Politics.” At night, the library tent fills with music from his blues CDs. When the “inevitable” order to leave McPherson Square comes, “I’ll chain myself to that shelf,” he says.

“We’re lucky here,” says Lisa McCracken, 35, a graduate student in library sciences who helps organize the titles. “In New York, they’re not allowed to put up structures.” She recently returned from the far larger library at Occupy Wall Street with several copies of Jamie Court’s “The Progressive’s Guide to Raising Hell.” She’d like to add more items about organizing grass-roots campaigns, or what she calls “ ‘now what?’ books.”

The collection of about 800 titles is hard to categorize. There are several Bibles, along with 13 volumes of “The Library of Curious and Unusual Facts.” Patrons can choose from David Baldacci’s thrillers, Dante’s “Divine Comedy” or Carl Sandburg’s poems. The shelves are no less eclectic than the people who mill in and out — some college students, some homeless. Louis Menand’s “The Metaphysical Club” sits peacefully next to romance novelist Nora Roberts’s “Blue Smoke.”

“I’ve always been fascinated by free-range libraries,” McCracken says. “What can we do to serve the community and the kids out here? And a library is familiar. People feel safer stopping by here for information.”

The rules are simple: no library cards, no due dates. “It’s take a book, leave a book,” McCracken says. “You give people a little bit of a guilt trip: ‘We’re trusting you to do right by this book.’ ” Many books never come back.

“We’re planning a field trip to the Library of Congress,” Gustavson says. “I’m dying to see that.”