Christine Gale Reynolds worked at the public library in Yosemite National Park before it closed in March when California Gov. Gavin Newsom issued shutdown orders to slow the spread of the coronavirus. So, she filled the back of her car with donated books and began her own mobile library.

"I know this may not be legal, conventional, or ethically sound, and yet it has worked for many, and I feel of use," she said. She physically distances while making her stops and sanitizes the books.

Across the United States, volunteers are reporting a jump in little free libraries as readers look to pass the...