You've probably seen them, even if you don't think you have: the little boxes that are part literacy outreach, part public art, part civic engagement project. Since 2009, Little Free Libraries have been popping up in yards around the country — and the world — like mushrooms after rain. Author Margret Aldrich's "The Little Free Library Book" reveals the history, the stories and the people behind the libraries. If you weren't already a devotee of this homegrown project, prepare to become one.

The premise is simple: A Little Free Library is a box made to hold books, where passersby are invited to take one, leave one, or just stop and browse. The owner (or "steward," in library parlance) fills the box with books, sometimes on a theme, sometimes just assorted castoffs from a home bookshelf, the collection changing over time. The library's physical housing can be as diverse and varied as its contents. Some are prefabricated and ordered from the Little Free Library website; some are made from repurposed household objects, castoff building materials, or most any other weatherproof material you might imagine. They're built by prisoners and retirees, professional craftspeople and families. They stand outside of homes, food pantries, churches, police departments and more. Aldrich's book is filled with stories of Little Free Libraries around the world: heartwarming vignettes, interviews and colorful photos that bring the projects to life on the page.

More than just a spot to trade books and maybe meet a neighbor, the libraries function as pint-sized social justice projects, bringing books to communities where they're lacking, a way of stemming the tide of illiteracy one volume at a time. Aldrich manages to take despairing statistics about book-poor communities around the nation and the world — areas with rampant adult illiteracy, or no public library or bookstore for miles — and shine a bit of optimism on them by profiling the people and organizations building Little Free Libraries in these spots where they're needed most.

"The Little Free Library Book" is both an entertaining read and an inspiring one. Aldrich writes with a clear, journalistic ease. The prose is informative but shot through with an obvious affection for her subject. And it's hard not to feel likewise for this project; with a relatively minimal outlay of time and money, anyone can become a part of a growing international movement to make the world a better place through books. Here's hoping that more than one reader of "The Little Free Library Book" will be moved to create a Little Free Library of his or her own, perhaps even buying an additional copy of Aldrich's book to place within its inaugural collection.

Emily H. Freeman is a writer, and a teacher of writing, in Missoula, Mont.