We know Ronald Reagan could write. There is a large volume of his correspondence with Americans he had never met but wanted to touch with a few personal lines of support or commiseration. He wrote the majority of his speeches and his weekly radio addresses, too. As an actor, he knew the cadences that would work and that couching an argument in an anecdote was far more potent than mere facts and figures.

We know he was an inveterate writer. Every day he wrote to expiate his thoughts. Even when he retired, he spent several days a week in his office in Century City, penning a few lines to well wishers. The act of writing was a pleasure to him, a soothing alternative to his other passion, reaching out to people in person.

When it became known that as president Reagan had written a personal diary in his own hand, bound in leather with the presidential seal on the cover, the sale of the rights to “The Reagan Diaries” (HarperCollins, 784 pages, $35) became a publishing sensation. At last we would discover the innermost thoughts of a man whose intense reserve had shut out his colleagues, his official biographer, his two wives, and even his children.

Perhaps the expectations were too high. In the hands of British politicians Richard Crossman and Alan Clark, diaries have shone a light into the recesses of government while offering a rare insight into how personalities interact under pressure. The prospect of a rare glimpse into Reagan’s mind as he faced Mikhail Gorbachev across the negotiating table or as he confronted the truth of his subordinates’ insubordination during Iran-Contra made the mouth water.

Now that the Reagan diaries are published, they barely live up to their great expectations. They provide some windows into his private world and offer hints about his feelings when events began to get on top of him, but in his diary entries, as in his life, the mind of the Great Communicator remains elusive.

He displays a touching devotion to Nancy (“What if she’d never been born. I don’t want to think about it” and “Saw ‘Mommie’ off for London & the Royal Wedding. I worry when she’s out of sight 6 minutes. How am I going to hold out for 6 days. The lights just don’t seem as warm & bright without her.”) and a candor about his recalcitrant children (“I told [son Ron] not to talk to me that way & he hung up on me. End of a not perfect day.”) but otherwise he mostly stays in the shallow end of his thoughts.

Take this, his bland account of a unique, historic, and pageant-laden visit to Britain arranged by Margaret Thatcher as a reward for his help during the Falklands war: “Helicoptered to Windsor Castle. This was a fairy tale experience. Black tie dinner with the Queen and Prince Phillip [sic] plus family — the Queen Mother et al. The next day a ride with the Queen. Nancy and the Prince did a carriage of 4 (horses). We then left for London — I addressed Joint session of Parliament. First Pres. Ever to do so. Lunch with P.M. Thatcher then back to Windsor for a White tie dinner.”

Even when in the heat of a crisis, his record is surprisingly cool, at one remove from reality. When a car bomb killed Americans at an air base in Germany, he wrote, “D—n terrorists—h—l is too good for them.” (His redactions.) And he displays a singular lack of resolve when terrorists bomb the American Embassy in Beirut in September 1984. “I’ve told George S. [Shultz] to let Syria know that we are convinced this couldn’t happen without their tacit approval & we d—n well will keep this in mind if it continues to happen.”

His character assessments are cryptic to the point of useless in gauging what he really thought. “[Ariel] Sharon is the bad guy who seemingly looks forward to a war.” On Margaret Thatcher, his closest ally: “I’m a great admirer of hers.” On the death of Fred Astaire: “He was a truly wonderful man.” And wishful thinking on Fidel Castro: “It’s just possible we could talk him into moving back in to this hemisphere.”

Reagan rarely exposes himself when under intense scrutiny, though his ability to live in a state of denial is fascinating to observe. So he writes that the Iran-Contra affair is largely a put-up job — “The whole irresponsible press bilge about hostages & Iran has gotten totally out of hand.” — followed by a terse account when the full truth of Oliver North and John Poindexter’s activities are explained to him. He does not mention his televised semi-apology to the American people for misleading them and himself. Only when the Reykjavik nuclear disarmament talks break down does he offer an illuminating flash: “I was mad — [Gorbachev] tried to act jovial but I acted mad and it showed.”

More telling than the conduct of policy, which is mostly adumbrated notes intended perhaps as an aide-mémoire for his memoirs, is his White House routine in which he kept strict office hours whenever possible.

He spent hours watching tennis or lying by the pool, apparently doing and thinking nothing, and each night, with Nancy by his side, he treated himself to a movie. He kept up with his old Hollywood pals, even when they became morbid reminders of good times past. His entry for September 18, 1982: “We watched the tapes of Princess Grace’s funeral.” When Rock Hudson dies from AIDS in Paris, he expresses surprise at the discovery the actor was perhaps homosexual.

But what emerges above all from these entries is how sweet a man Reagan remained, despite a long life in the caustic worlds of Hollywood and politics. “I spent my time picking up acorns — a big bag of them,” he wrote one day. “I’m going to give them to the squirrels outside the Oval office.”

Mr. Wapshott’s joint biography of Reagan and Thatcher will be published by Sentinel in September.