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She also provides an occasionally searing look at how hard it is to be a teenager, especially female, with the pressure to be liked and to belong snuffing out one’s better instincts.

“To improve your image, you made yourself more stupid and less kind,” she writes, after young Petra has betrayed a friend. “The shutting down of some vital part of yourself, just so you could be included on a shopping trip into town, not have to sit on your own at lunch or have someone to walk home with.”

The idea for the book came to Pearson years ago in Norway, during the publicity tour for her first book. At dinner with some women, the subject of teen crushes came up, and Pearson related hers, on Cassidy. A woman who’d been quiet the entire meal said, in a strikingly serious tone: “But he was mine.”

Pearson felt she was onto something. She went onto eBay and started acquiring Cassidy memorabilia. A tiny office at the top of her home in Cambridge, England, became Cassidy central. It looked, her husband commented, “like a serial killer’s lair.”

The book took a long time, long enough that she was sued by publishers for non-delivery. The suit has been settled now, Pearson says, and movie rights are already being actively discussed.

Though Cassidy’s representatives had given the book a green light, Pearson never knew how he felt about it _ until Thursday, on the “Today” set. She told him she hoped he didn’t hate her for writing the book. On the contrary, he gushed.

It wasn’t the first time the author had met her subject. In 2004, she was assigned by the Daily Telegraph’s Saturday magazine to interview Cassidy, then 54, in Florida, unrelated to the book idea. She appends the transcript to the end of her novel.

At the end, the reporter confesses to the teen idol that she had sacrificed her fashion sense as a girl to swathe herself in his favorite color, brown.

He laughs at her teenage cluelessness.

Allison, it was all made up!” he says. “Do I look like someone whose favorite color is BROWN?”