Having spent much of her life on its shores and water, Mikaela Casey knew Lake Minnetonka had nice bays, good restaurants and beautiful sunsets. One thing it didn’t have was its own children’s book.

That thought occurred to Casey one evening while in a boat on the lake, doing homework for a children’s literature class she was taking at the University of Minnesota. “You know, I should probably write that,” she recalls thinking.

A couple of years ago, armed with an English degree, Casey started writing.

The result is “Lenny the Loon: An Adventure on Lake Minnetonka,” a hardcover picture book that Casey, now 28, self-published and launched in November. The locations in the book are colorfully (and accurately) depicted by Yana Popova, a London-based illustrator who Casey found on Facebook and paid $3,000.

To say a book about a loon has been flying off the shelves would be, well, a groan-worthy bit of wordplay. But within weeks, Casey sold the 750 copies needed to earn back the $15,000 she invested in the book, after it was rejected by a number of publishers.

With help from book-signing events at retail spots around the lake and energetic marketing on social media, “Lenny” now has sold about 1,200 copies, far exceeding the author’s expectations.

“I really hoped I would break even,” Casey said.

The book has proved popular as a gift for kids and a memento for lake enthusiasts. Many people have bought four or more copies at a time, she said.

At Excelsior Bay Books, where Casey recently held a signing event, co-owner Ellie Temple estimated that “Lenny” has probably sold more than 100 copies, making it the shop’s bestselling children’s book this fall. (The best ever? No, Temple said. “Harry Potter” set the bar pretty high.)

Casey grew up in Mound, a couple of blocks from the lake. At 13, she started working at Al & Alma’s Supper Club, a 62-year-old local institution famous for its dinner cruises. She started serving food and drinks on the cruise boats and got to know her way around Minnetonka’s sprawling waters, eventually earning a captain’s license and driving the boats as well.

To learn how to write children’s books, Casey read a couple guides and concocted a plot featuring a loon, Minnesota’s state bird and her own favorite.

Writing in verse, she found that keeping the language and ideas simple enough for small children was harder than it seemed. Substituting a shorter word for a long one meant finding a different word to rhyme. “Lenny” went through 12 drafts.

The book tells the story of Lenny, a loon who lives on Lake Minnetonka and encounters a walleye named Wally who wants to move to Mexico. Lenny agrees to guide Wally to Grays Bay, where fish can enter Minnehaha Creek and swim to the Mississippi River and eventually the Gulf of Mexico.

OK, so walleyes don’t live in saltwater, but this isn’t an ichthyology textbook. Fish don’t talk, either.

Lenny and Wally traverse the lake, past Mound and Excelsior and Wayzata, taking in its water skiers, cruise boats and restaurants. The book name-checks Cook’s Bay, Seton Channel, Spirit Island and other lake features, giving readers a lesson on the lake’s geography. It includes a map of the whole lake marked with the characters’ route.

At one point, the two aquatic friends pause to admire a sunset and the text turns philosophical: “It’s hard to see the beauty of a place/ When you go through life as if it’s a race.”

As they approach the creek, Wally starts to look downcast. “[W]hen Lenny looked at Wally/ He saw a frown/ ‘What’s wrong, my friend?/ You’re about to leave town!”

Spoiler alert: There’s a happy ending.

“Lake Minnetonka is a place of magic,” the book concludes. “Missing out on the fun would be ever so tragic!”

That sentiment reflects Casey’s own feelings. Though now living in Minneapolis, she has a full-time job as a personal assistant to a family living on the lake. She still considers the lake her home.

“It’s carefree, a place when you can leave your troubles on the shore and enjoy yourself,” Casey said. “I think the best word for it is tranquil.”