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I'm assuming that you are not the person who placed that "Boston" cassette into a neighborhood Little Free Library recently, but the offense does provide an opportunity to reiterate a message my poor children hear with annoying frequency:

This doesn't belong here!

There are few additions to our big and increasingly crass world that are as inspiring, community-building and democratic as the ever-expanding collection of Little Free Libraries popping up on tree-lined streets, sidewalks in front of shopping centers and inside tree trunks in Nowhere USA.

In just seven years, the phenomenon that began in Wisconsin with a single miniature schoolhouse filled with books has blossomed to nearly 36,000 Little Free Libraries (LFL) around the world.

So, can we please not muck it up?

I don't imagine I'm the only person noticing that a LFL here and there is starting to look a bit like that shelf in our mudroom, collecting miscellany that we don't want but can't quite throw away should we need it when we retire, divorce or die. Instructions for that Ikea dresser. A bestselling bodice ripper from 1997. Religious tracts. Phone books.

Others are so stuffed with books from someone's basement that no hopeful small child can get her hands around anything.

Resist!

We are all stewards in the upkeep of these magical book exchanges. I'm happy to be the annoying mother figure who reminds the masses just how lucky we are, although I'm in very good company.

Margret Aldrich also marvels at the fact that since 2009, when Todd Bol built the first LFL in Hudson, Wis., the idea has spread to all 50 states and more than 70 countries. That means more than 35 million books in the hands of grateful readers.

"I love that this one idea — this one really good idea — has spread around," said Aldrich, a Minneapolis resident and author of the charming, photo-filled homage to the concept, titled "The Little Free Library Book" (Coffee House Press, 2015).

"We joke that Todd is the Johnny Appleseed of books."

Aldrich, who has two young sons, sees many reasons for their astounding popularity. "They fill a need for community that a lot of us feel we've lost," she said. "You share a book and bump into a neighbor."

The thrill of finding something unexpected, such as a title you might never have considered, draws us in, too. "Little Free Libraries," she said, "give you that fix."

Her family's yellow-and-white library sits at the foot of the couple's expansive and hilly south Minneapolis front yard.

"Every time I look out our big picture window and see someone at our Little Free Library, I cheer," Aldrich said. "It's a wonderful feeling."

She makes sure that others know the feeling, too. Aldrich keeps books donated from fellow parents and librarian friends in the trunk of her car. When she sees a library in need of love, "I'll pull over and fill 'em up," she said.

Recently, she visited Cleveland, where LFLs are being built to support literacy, and happily went on a "restocking field trip."

Anybody can build one

While Aldrich's book reveals stunning examples of creativity and artistry in LFL design — a rooster and rocket ship, a Volkswagen bus and a Dr. Who TARDIS, birdhouses and a Victorian mansion — simple designs also earn oohs and aahs because it's what's inside that counts.

"Part of the beauty," Aldrich said, "is that there are no rules about what a library should look like."

The LFL website — littlefreelibrary.org — takes you through the simple steps of building one, from identifying a good location, to ideas for being a good "steward," to registering your LFL and getting the word out.

To get the word out further, LFL founders are throwing the first Little Free Library Festival on May 21, with a day filled with family-friendly, literature-focused events for adults, kids and dogs.

In addition, 100 completed LFLs will be given away to members of "communities of impact" who have less access to bookstores and books, said Tony Bol, brother of Todd Bol and coordinator of the free festival (apply at littlefreelibrary.org/festival).

"Minnesota has a legacy of doing good and can be extremely proud that it has done what it has done here and elsewhere," he said.

Fortunately for me, he, too, has a neat streak. His LFL has three shelves; children's books on the bottom, community information in the middle and adult books on top.

"I take out stuff I just don't like," Tony Bol said with a laugh. "Sometimes, I'll get rid of it — or put it in someone else's library."

gail.rosenblum@startribune.com 612-673-7350 • Twitter: @grosenblum