You’ve probably never heard of Todd Bol, who died on October 18 at the age of 62. But he was one of the most significant social entrepreneurs of this century. Bol invented the Little Free Library whose tiny little bookshelves are in front yards across North America, inviting people to take a book for free and place a book in the bookcase in return.

In Bol’s Washington Post obituary, Harrison Smith notes that Little Free Libraries have only been in existence for a decade. But they are now part of urban life everywhere, with 75,000 of them around the globe.

“They stand inside Los Angeles police stations and at New York City subway stops,” Smith writes, “at prisons in Wisconsin and hospitals in Ireland, at a refugee camp in Uganda and a schoolyard in South Sudan.”

Bol graduated from the University of Wisconsin (River Falls) in 1979. He spent the first years of his career working for 3M, and then created the Global Scholarship Alliance, which helped foreign students get scholarships to study as nurses in the United States. In 2008 he was pushed out of the Global Scholarship Alliance and was looking for a new venture.

He decided to make a building that looked like a little schoolhouse but could be placed on a pole and put it in front of his house, filling it with books. People liked it, so he built a second one. And then, working with his friend Rick Brooks, he decided to make more of them. He ultimately vowed to build 2,540 of them, to outdo Andrew Carnegie’s 2,535 public libraries. Bol met his goal within four years.

By 2011, Little Free Libraries were all over Minneapolis and St. Paul. “They look like large birdhouses and act like water coolers,” Minneapolis Star-Tribune reporter Bill Ward noted.  But word about the Little Free Libraries spread, thanks to social media and a booth at the Minnesota State Fair.

Little Free Libraries spread organically. There was no national campaign to promote them. People heard about them, and then either sent off $150 to Little Free Library headquarters for a finished version (made by Amish craftsmen in rural Wisconsin) or $40 for the blueprints. People made their own versions, including some that looked like Doctor Who’s TARDIS.

Sometimes people just heard about Little Free Libraries, took the idea, and ran with it. Rick Brooks, for a profile in Beloit College Magazine discusses one woman from Ghana who saw the Little Free Libraries website and ended up building 42 Little Free Libraries in Ghana and Nigeria.

In recent years, some social entrepreneurs have taken the Little Free Library idea and extended it to other things. In Fayetteville, Arkansas, Jessica McClard in 2016 started the first Little Free Pantry outside the Good Shepard Lutheran Church. She told ABC News that she makes sure to check the pantry every day because “it turns over in about 30 minutes” after the day’s supply of canned food is placed in the pantry.

In Lansing, Michigan, Adriana Flores, who recently got a master’s degree from the Michigan State University School of Social Work, created the E² Box to supply toiletries and feminine hygiene products (tampons, sanitary pads) not covered by government assistance. Why “E Squared?” Because, Flores says, that stands for “empathy and equity.” Right now there is only one E Squared Box, but Flores hopes to create more.

One barrier that stands in the way of creating Little Free Libraries are planners who object to people building them without getting permits. Writing in The Atlantic in 2015, Conor Friedersdorf discusses how planners in Los Angeles and Shreveport tried to block the creation of Little Free Libraries because the residential streets where people were placing Little Free Libraries in their front yards weren’t zoned for them. Ultimately the planners gave permission, but Friedersdorf is right to argue that objecting to the creation of a Little Free Library simply because it does not fit a zoning code is ridiculous, given that a Little Free Library is scarcely larger than a birdhouse.

Todd Bol did a good deed when he thought up the Little Free Library. The success of the Little Free Library movement shows how a social entrepreneur who comes up with a simple idea can make our communities better. As Jacqueline Pfeffer Merrill once noted in these pages, “each little library success is an instance of ‘philanthrolocalism,’ which brings together a local community of donors and beneficiaries… it’s a global success achieved by thousands of small-scale efforts in local communities and neighborhoods.”