HUDSON, Wisconsin (AP) ― It started as a simple tribute to his mother, a teacher and bibliophile. Todd Bol put up a miniature version of a one-room schoolhouse on a post outside his home in this western Wisconsin city, filled it with books and invited his neighbors to borrow them.

They loved it, and began dropping by so often that his lawn became a gathering spot. Then a friend in Madison put out some similar boxes and got the same reaction. More home-crafted libraries began popping up around Wisconsin’s capital.

Three years later, the whimsical boxes are a global sensation. They number in the thousands and have spread to at least 36 countries, in a testimonial to the power of a good idea, the simple allure of a book and the wildfire of the Internet.

“It’s weird to be an international phenomenon,’’ said Bol, a former international business consultant who finds himself at the head of what has become the Little Free Libraries organization. The book-sharing boxes are being adopted by a growing number of groups as a way of promoting literacy in inner cities and underdeveloped countries.

Bol, his Madison friend Rick Brooks, and helpers run the project from a funky workshop with a weathered wood facade in an otherwise nondescript concrete industrial building outside Hudson, a riverside community of 12,000 about 30 kilometers east of downtown St. Paul, Minnesota. They build wooden book boxes in a variety of styles, ranging from basic to a miniature British-style phone booth, and offer them for sale on the group’s website, which also offers plans for building your own. Sizes vary. The essential traits are that they are eye-catching and protect the books from the weather.

Each little library invites passersby to “take a book, return a book.’’

Educators in particular have seized on the potential of something so simple and self-sustaining.

In Minneapolis, school officials are aiming to put up about 100 in neighborhoods where many kids don’t have books at home. A box at district headquarters goes through 40 books a day, serving children whose parents come to register them and adults who come to prepare for high school equivalency tests.

“I absolutely love them,’’ said Melanie Sanco, the district’s point person on the effort. “It sparks the imagination. You see them around and you want one. ... They’re cute and adorable.’’ Kids who have books stay in school longer, she said.

Bol and Brooks, who runs outreach programs at the University of Wisconsin, see the potential for a lot more growth. At one point, they set a goal of 2,510 boxes ― surpassing the number of public libraries built by philanthropist Andrew Carnegie. They passed that mark this summer.

The Rotary Club plans to use the book boxes in its literacy efforts in the west African nation of Ghana. Books for Africa, a Minnesota-based group that has sent over 27 million books to 48 countries since 1988, recently decided to ship books and little libraries to Ghana, too.

The groups are working with Antoinette Ashong, a pro-literacy activist and headmistress of a girls’ school in the capital of Accra. “I want to spread reading in Africa, which is a problem because in Africa it is very, very difficult to get books to read,’’ Ashong said in a Skype interview. She has already put up 45 boxes in poor neighborhoods.

Most of the nonprofit’s money comes from the sale of pre-built little libraries, which cost from $250 to $600, and a $25 fee to register a library on the organization’s web site. The AARP Foundation has also provided a $70,000 grant as part of a new program to provide book boxes for seniors and kids to read to them.