As Fonzie, Henry Winkler was the definition of cool. Black leather jacket. Motorcycle. Girls. But when he was growing up on the Upper West Side, Winkler’s youth wasn’t exactly filled with happy days.

He had dyslexia and didn’t know it, Winkler tells The Post – and he had so much trouble reading that grown-ups constantly told him he wouldn’t amount to anything.

Boy, were they wrong. At 57, the onetime “Happy Days” star – now appearing in the hit film “Holes” (he plays young hero Stanley’s dad) – is the proud author of a series of new kids’ books about a dyslexic like himself: “Hank Zipzer: The Mostly True Confessions of the World’s Best Underachiever.”

Due out Monday, the first two are “Niagara Falls, or Does It?” and “I Got a ‘D’ in Salami.”

They’re the kid-friendly tales of fourth-grader Hank Zipzer, a frequent visitor to Principal Love’s office, sent by his humorless teacher, Ms. Adolf, for one infraction or another – all because Hank has a hard time reading and doing his schoolwork.

Hank’s childhood is a lot like Henry Winkler’s was, down to the apartment building on 78th Street, between Broadway and Amsterdam, just down the street from Winkler’s alma mater – PS 87, where Zipzer and his best friends Frankie and Ashley go.

Turns out, Winkler even had a teacher named Ms. Adolf there. Would she be excited if she knew her onetime troubled student was a published author?

“She’d scour the book, looking for mistakes,” he laughs.

“I was always getting in trouble with her,” Winkler continues. “I used to be the class clown. So, if she was reading a story to the class, I was acting it out.

“I got detention. I got letters home.

“And I wanted to be like a great student. I reinforced every hole on every piece of paper. I had every pencil sharpened . . .

“I just couldn’t do it and I had no idea why.”

Winkler’s school daze didn’t end at PS 87. He was forced to take the same geometry course four straight years in high school (at the McBurney School for Boys), had to take summer classes after his senior year, and wasn’t allowed to attend graduation – he got his diploma in the mail instead.

So imagine how happy – and surprised – Winkler was when, fresh out of Emerson College, he was accepted into the Yale School of Drama.

“I threw the windows open in my Commonwealth Avenue [Boston] apartment and yelled out: ‘I got into Yale!’ ” he recalls.

Even so, his parents weren’t happy. They wanted Henry to go into his father’s business – importing hardwoods.

It wasn’t until he became a star on “Happy Days” that they were proud of him.

They came along when he presented Fonzie’s leather jacket to the Smithsonian in Washington – and when Winkler was invited to the Carter White House, his dad told him to bring cake. (They had cake at the White House, Henry told him.)

But despite his success – he’s now a producer as well as an actor – Winkler still has trouble reading.

“I really have to concentrate a lot harder,” he says, “and I let scripts pile up.”

Nor is he alone. His stepson Jed, now grown, was in the third grade and could hardly write.

“If he had to write a book report, it was usually a paragraph,” Winkler says. And in frustration, “he pushed the pencil through the paper.”

When Winkler and his wife, Stacey, took Jed for help, Winkler realized the boy was having the same problems he had.

“Every single thing the therapist said to Jed, I went, ‘Oh my God. That’s me.’ So at 31, I realized it wasn’t that I was stupid.”

He says he wants kids with reading problems – and one out of five children have some kind of learning challenge – to know they’re not alone.

If they can’t get through his books, co-written with Lin Oliver, he suggests parents read along with them.

As for his own challenge: “All I want is to make kids laugh, and hope they say, ‘I really had a good time’ – I want to read another.’ “

Signs of trouble & how to help

How do you know if your child is at risk for a reading problem? Linda Selvin, executive director of the New York Branch of the International Dyslexia Association, offers a few signs to look for:

Beginning to talk later than normal; vocabulary slow to develop.

Having difficulty rhyming words.

Trouble repeating the alphabet.

Inability to retrieve the right word.

Problems with pronouncing words like refrigerator or spaghetti.

If you’re concerned, get your child evaluated, Selvin says, adding, “Don’t let someone tell you ‘They’ll grow out of it.’ “

School districts can set up an evaluation; some hospitals have learning centers which may be covered by insurance. First, talk to your child’s teacher or principal.

“Don’t take no for an answer,” Selvin says. “It’s important to identify any problem early – and get help.”