PORTLAND, Ore. — A homeless man named Daniel was engrossed in a Barbara Kingsolver novel when his backpack was stolen recently, and Laura Moulton was determined to set things to right.

Ms. Moulton, 44, an artist, writer and adjunct professor of creative nonfiction, did not know Daniel’s last name, his exact age, or really even how to find him — they had met only once. But she knew the novel, “Prodigal Summer,” and that was a start. So, armed with a new copy of the book, off she went.

Such is the life of a street librarian.

This city has a deeply dyed liberal impulse beating in its veins around social and environmental causes, and a literary culture that has flourished like the blackberry thickets that mark misty Northwest woods. It is also one of the most bike-friendly, if not bike-crazed, urban spaces in the nation, as measured by commuters and bike lanes. All three of those forces are combined in Street Books, a nonprofit book service delivered by pedal-power for “people living outside,” as Ms. Moulton, the founder, describes the mission.

“Is Daniel around?” she asked a patron, Laura King, having just trundled up on the Street Books three-wheeler on a recent afternoon for a stop near the Willamette River northeast of downtown.