Everyone knows you’re not supposed to judge a book by its cover, but many of us do it anyway. In the case of Joan Bauer’s 14th novel, RAISING LUMIE (Viking, 288 pp., $16.99; ages 10 to 13), you might see the jacket photograph of a fuzzy yellow Labrador retriever puppy and assume you’re in for a cozy, heartwarming tale. You would be wrong. Sort of.

Bauer’s story of a 12-year-old girl training a guide dog contains plenty of touching and gratifying moments. In fact, it’s so jam-packed with inspiring canine cuteness I kept having to excuse myself to dole out treats to my exuberant mutt. But “Raising Lumie” is a wolf in sheep’s clothing — a sad story wrapped in an uplifting one whose core you never quite forget, no matter how tickled you are by its woolly exterior.

Like Walt Disney, Bauer wastes no time dispensing with parents. Olive Hudson doesn’t remember her mom, who died in a car accident when Olive was 2; her dad, a plumber named Joe, has just died of cancer. (To her credit, Bauer avoids euphemisms like “passed.”) Olive’s grandmother is alive and appears to have all her marbles (as my grandmother used to say), but the orphaned middle schooler is dispatched to a different New Jersey town to live with Maudie, a half sister she barely knows. Olive is a list-maker — it’s how she keeps grief at bay — so in her honor here are some facts about Maudie: She works as a graphic designer. She’s engaged to Roger, whose best asset is a fancy car. She’s unflappable and unfailingly cheerful.

Either Olive is the most amenable adolescent on the planet or the ones in my orbit are just unusually crotchety. She’s sad to leave everything and everyone she has ever known, but also totally game to relocate with Maudie to the Stay Awhile, a big yellow boardinghouse shared by a cast of charmingly quirky residents including a free-range rabbit named Bunster. Olive even performs a plumbing miracle mid-move; she fixes a spraying faucet in the home of a complete stranger, leaving behind a note: “This repair is in honor of Joe Hudson, the best plumber in America.”

Very young readers may appreciate Olive’s “Leapin’ lizards!” approach to seismic change, but kids her own age may not buy it — at least not this summer, when it seems as if loss is everywhere.

Lumie is by far the best character in this book. She’s the 8-week-old puppy Olive meets when she joins Maudie for her first day at her new job. (Puppies, cinnamon buns and young relatives welcome? Where do I apply?) According to Olive, Lumie is “the most adorable dog in North America … a little fur machine that is 150 percent ON.” And she just happens to be hanging around the office in preparation for her future role as a guide dog.

By the time Lumie’s designated “raiser” gets waylaid at a funeral in Germany, Olive is the obvious candidate to work with the pup. Everyone at the Stay Awhile is on board, including Bunster. Olive tells Lumie, “I’m here to help you get ready to do big things in the world” — and she certainly holds up her end of the bargain. If Bauer’s book is made into a movie, this will be the montage scene: girl and dog exploring the neighborhood, practicing leash skills, delighting friends and strangers. When Olive starts seventh grade, Maudie sends regular newspaper-style dispatches about Lumie’s behavior; they’re an excellent icebreaker for Olive to read aloud in the school cafeteria.

Of course this is a temporary arrangement. The idea is for Olive to pass Lumie along to an owner whose world she’ll open in the same way she has opened Olive’s. This plan has hitches — some more upsetting than others — but Olive is a survivor, which makes her fun to root for, and Lumie is as dear to the reader as any human.

“Raising Lumie” is a touching, optimistic story about decent, well-intentioned people, which should be enough to earn it a spot on the shelf of any tween dog-lover. Maybe I’m getting cynical in middle age, but I kept unzipping Bauer’s carefully spun fleece and finding myself face to face with the aforementioned wolf. I wonder, would a puppy and a pack of strangers really take away the pain of Olive’s loss? Would they even make a dent? Hard to say. But who wouldn’t want to live in a world where the answer is yes?