(CNN) Today is its day! "Oh, The Places You'll Go!" written and illustrated by Theodor Geisel, better known as Dr. Seuss, debuted 25 years ago on January 22, 1990.

Since then, millions of high school and college grads have heard the same zealous affirmation, a not-so-gentle reminder to live up to their potential -- "KID, YOU'LL MOVE MOUNTAINS!" -- mixed with warnings about loneliness and uncertainty.

It's a children's book, as most Dr. Seuss books are, but its messages have resonated through a quarter-century because adults love it too, said Philip Nel , director of the program in children's literature at Kansas State University.

Children's books "possess an enormous amount of wisdom for the adults mature enough to recognize it," he said.

Here are a few facts about "Oh, The Places You'll Go!" and its legacy.

Dr. Seuss knew it would be the last book he published

Geisel was writing and illustrating "Oh, The Places You'll Go!" in the final years of his life, as he battled cancer. Other Seuss books have emerged since his death in 1991, but this was the last he had a hand in.

"It was his swan song, and he knew it was his swan song," Nel said. "It's a career summation in a lot of ways, both visually and morally, in the messages and the images."

Though it presents plenty of Seussian optimism -- "And will you succeed? Yes! You will, indeed! (98 and 3/4 percent guaranteed.)" -- it references dark times and stumbles: "I'm afraid that some times you'll play lonely games too. Games you can't win 'cause you'll play against you."

Geisel wrote from experience. After a successful career in advertising, his first book, "And to Think That I Saw It on Mulberry Street," was famously rejected multiple times. Some projects in print and on screen simply never took off. In his personal life, he faced illness and the suicide of his first wife.

"The darkness of the book is what makes the optimism credible," Nel said.

Geisel was known to be shy but would be "tickled" to know that his words still have meaning now, said Susan Brandt, president of licensing and marketing at Dr. Seuss Enterprises. He was deliberate about what his last words to the world would be.

"You can feel him saying, 'Wait, I have one more thing to say,' " she said. " 'Listen to me; this is important.' "

It's the top-selling Dr. Seuss book

"Oh, The Places You'll Go!" rose to the bestseller list shortly after it was released in 1990 and continues to pop up there most every spring, as high school and college grads transition to a new phase of life. Twenty-five years and more than 10 million copies later, it's the top-selling Dr. Seuss book.

The company it keeps at the top? Beloved early-reading books like "Green Eggs and Ham" and "The Cat in the Hat," perennial holiday read "How the Grinch Stole Christmas!" and books with a message -- and a movie -- like "The Lorax" and "Horton Hears a Who!"

Its success isn't exactly a surprise, said Mallory Loehr, vice president and publisher of Random House Children's Books. It was published the year she began her career at Random House, and she remember the buzz around the book even then.

"People knew it was special (in 1990)," Loehr said. "But 10 million is crazy."

The main character is You -- and you, and you, and you

Passages in "The Cat in the Hat" and "The Lorax" both slip into the second-person to make a point to readers, but Geisel made the main character of "Oh, The Places You'll Go!" a person called You, who conveniently happens to be, well, you.

"That makes the book about the reader and puts the reader in the position of the character," Nel said. "It's a motivator. See yourself as the character; take action."

Dr. Seuss had already been a major player in children's literature for decades, but this was an early entry into a burgeoning category: children's stories that appeal to adults, Loehr said.

"This is almost a self-help book. People forget that it has pages that really acknowledge that things will be tough some times," she said. "Sit down; read this book. It will be OK."

As Geisel himself put it, "I never write for children. I write for people."

The number of "Places You'll Go" is growing

What started as a beloved graduation gift has spread into sort of an all-purpose greeting card, shared with a personal inscriptions during any period of transition, from baby showers to big moves. This month, Random House will publish a 25th anniversary edition of "Oh! The Places You'll Go!" This year, it will release a baby-fied version, "Oh, Baby, the Places You'll Go!" and a full-color journal filled with quotes from the book.

On March 2, Geisel's birthday, Dr. Seuss Enterprises will launch KidsMovingMountains.com , which calls for children age 5 to 18 to share stories of how they're "moving mountains" in science, technology, engineering, arts and math. They'll award scholarships and trips to some top students.

There's never been a film version, but Kansas State University's Nel said Geisel hoped there would be an adaptation -- maybe something "famous as famous can be, with the whole wide world watching you win on TV."

For his part, Nel said, it's a remarkable book, but he hasn't given it as a graduation gift.

"It's not that it's not good," Nel said. "It's that everybody I would give it to, someone has has already given it to them."