Queen Elizabeth II with Prince Charles on the grounds of Balmoral Castle in 1952. (Hulton Deutsch/Corbis via Getty Images)

‘Poor Charles.”

That’s what Sally Bedell Smith kept hearing from everyone as she worked on her new book about the British monarch-in-waiting. Charles Philip Arthur George has been heir to the British throne for 65 years: His mother became queen when he was 3 years old, and she is still going strong at 90. He has spent his entire life waiting for his one and only job.

Overshadowed in turn by his mother, his first wife, and now his two sons, he’s best known as the prince who married Diana and was a terrible husband. When she died in 1997, the narrative was all but set in stone: Charles was dull, stoic and not very sympathetic.

“The vision we all have of him is of this extremely buttoned-up stereotype — double-breasted suit encasing him — a stiff, an old fogey, the guy who ruined Diana’s life,” says Bedell Smith, who first met the prince 26 years ago. “I was so struck by how different he was: funny, informal, warm, with this in­cred­ibly sexy voice.”

Four years ago, the Washington-based author of biographies about Diana and the queen decided to tackle the man who would be king. Her 500-page book, “Prince Charles: The Passions and Paradoxes of an Improbable Life,” is not an authorized biography, but the palace assisted with access to public appearances, interviews and research.

It shows Charles as a royal son, a father, an activist and an eccentric. He owns shoes made from 18th-century reindeer skins. He is both very old-fashioned (he doesn’t use computers) and very modern (he is a lifelong proponent of conservation and sustainability). He is rich, but not above courting Americans to support his charities, including several wealthy patrons here in Washington.

At its heart, the book is the story of a sensitive, lonely kid and his quest to find purpose in his life. Temperamentally the opposite of his mother — she’s straightforward and unflappable — Charles has always been too emotional and too insecure for a life that demands a thick skin and personal sacrifice.

But what choice does he have? There is a sign in his dressing room at his country home, Highgrove: “Be patient and endure.”

“Prince Charles,” by Sally Bedell Smith. (Max Hirshfeld)

In the 1970s, Charles was the most eligible bachelor in the world. The tabloids breathlessly reported every date and scrutinized every girlfriend as a future queen. President Richard M. Nixon tried matchmaking for his daughter, Tricia, and seated Charles next to her at every event during the prince’s 1970 visit to Washington. (Charles was unimpressed, describing her as “artificial and plastic.”)

Everyone knows the story of his world-famous, ill-fated first marriage to Lady Diana Spencer. Bedell Smith explains why he proposed to a 20-year-old whom he barely knew. He had followed the advice of his confidant and mentor, Earl Louis Mountbatten, and had enjoyed affairs with women who were not, by the standards of the day, fit to be a princess. But Charles planned to marry by his 30th birthday, and he felt anxious and pressured when that date passed with no bride. When Diana set her sights on him, he married her in 1981 although he was not in love with her.

He was, however, crazy about Camilla Parker Bowles, whom he had met in 1972. She was irreverent, sexy, unintimidated and an ideal complement to the serious heir to the throne. The two had a six-month affair, but Camilla was besotted with her on-again, off-again boyfriend, Andrew Parker Bowles. She married the unfaithful charmer while Charles was away on naval duties, which stunned the prince. But the two remained friends and resumed their relationship in earnest about five years into his very unhappy marriage.

That love triangle ended in a messy and humiliating divorce, which painted a public portrait of Diana as the victim of an unfeeling royal family and of Charles as an insensitive jerk. The palace was in the middle of a cautious public relations rollout to introduce Charles and Camilla as a couple when Diana was killed in 1997. It took another eight years before they finally felt that it was possible to marry without endangering his claim to the throne.

In the meantime, he busied himself with dozens of causes, which he takes very seriously. “He has really labored to be admired and accepted for the things he has done rather than what he was born to be,” Bedell Smith says.

As Prince of Wales, Charles inherited the Duchy of Cornwall, which generates upward of $25 million a year in income for him, which pays for his household and staff and supports William and Kate and Harry. Despite his wealth, he has never had qualms about raising millions from American patrons for his charities. “Charles’s cunning in extracting money from eager benefactors was perilously intertwined with a weakness for the company and perks of the superrich,” Bedell Smith writes.

In 1997, Charles hired Robert Higdon, a Washingtonian who had worked for Ronald and Nancy Reagan and Margaret Thatcher, as executive director of the Prince of Wales Foundation. Higdon revamped it, expanded its charitable mission and persuaded couples to donate $20,000 each to hobnob with Charles at Highgrove and other royal palaces. The visits also became a vehicle for Camilla to launch an international charm offensive.

That effort was so successful that in 2008, Joe L. Allbritton sank $2.5 million into the development of Duchy USA, a line of products from the prince’s properties. After almost a year of planning, the project was abruptly canceled when the palace sold the worldwide rights to a British supermarket chain.

But all was forgiven — Allbritton and his wife, Barby, were invited to William and Kate’s wedding in 2011, and Allbritton loaned Charles his private jet for a quick trip to Washington.

Prince Charles with Prince Harry, center, and Prince William at a 2007 memorial service in London for Princess Diana, who died in 1997. (Tim Graham/Getty Images)

The queen turns 91 this month and is still deeply involved with her royal duties. She has gin with Dubonnet at lunch and a martini before dinner. Charles, now 68, holds the record as the longest heir-in-waiting and could easily go another decade before becoming king. (His maternal grandmother lived to 101.)

“The life that he’s led and the troubles and torments that he has had have, in a way, made it possible for William and Harry to lead much more normal lives,” Bedell Smith says. One royal adviser told her, “These are two guys on a raft who escaped from the shipwreck of their family and made it to the other shore.”

Talk of skipping Charles and giving the crown to William has subsided, which gives the young prince more time to enjoy a traditional (by royal standards) family life. Harry, who is fifth in line to the throne, probably has the best of both worlds — an incredibly tight relationship with his brother and enough fame and money to do pretty much anything he wants.

And Camilla? Time heals, or at least forgives. The woman once dubbed “the Rottweiler” has achieved grudging public acceptance. She has the full support of the queen (grateful that her oldest son is finally happy) and an easy camaraderie with William, Kate and Harry.

When the couple married in 2005, the palace tried to mollify Diana loyalists by saying that Camilla would be called “princess consort” when Charles becomes king. Now, it looks as if she may become queen after all. When asked about this during a 2010 interview with NBC, Charles stammered: “That’s, well . . . we’ll see, won’t we? That could be.”

But that, like everything else for Charles, is somewhere in the future.