People have been anticipating Bill Clinton's memoir ever since he left the White House in 2001. But that's nothing compared to the length of time historians have waited for a truly great presidential autobiography.

By consensus,"The Personal Memoirs of U.S. Grant" was the last. Twenty-one presidents have come and gone since 1885, the year Grant's book was published. Though four of those died in office (William McKinley, Warren Harding, Franklin D. Roosevelt and John F. Kennedy), more than a dozen ex-presidents have had the chance to write a truly compelling book. But historians say they're still waiting.

"There are not a lot of great presidential memoirs," says Lewis Gould, a presidential historian and author of"The Modern American Presidency.""In fact, I'd be hard-pressed to think of one that you'd want to take home and read over the weekend just for the sheer joy of it."

Adds Robert Dallek, a presidential historian and biographer of John F. Kennedy, Lyndon Johnson and Ronald Reagan:"There was only one (great presidential memoir), and that was Ulysses Grant. All the rest are dreary, overly partisan attempts at self-defense or self-justification. This doesn't make for great reading at all. It leaves out the flaws and weaknesses that make any human being interesting." Still, expectations are high for the Clinton memoir as the author begins a publicity blitz that starts with an hourlong interview with Dan Rather tonight on CBS'"60 Minutes." Presidents George H.W. Bush and Ronald Reagan published collections of their letters after they left the White House, but Clinton's will be the first presidential memoir since Jimmy Carter's"Keeping Faith: Memoirs of President," in 1982. There is a lot for Clinton to grapple with, including issues of substance, style and scandal from his colorful life in politics. And the news that the writing was done not by a ghostwriter but by Clinton, a man whose intellect is respected even by his fiercest critics, has political junkies anxiously awaiting the 900-plus-page book.

It will surely sell a lot of copies — it topped Amazon.com's best-seller list last week, long before Tuesday's official publication date — but if it's a strong critical success, it will surge toward the front of the pack of presidential memoirs. This leads to the question: What has prevented the most powerful men in the world from writing books that are of much use to historians, let alone of interest to the public?

Historian Richard Shenkman, author of"Presidential Ambition: How the Presidents Gained Power, Kept Power and Got Things Done," says outstanding presidential memoirs are scarce because writing one is at odds with the mindset of most legacy-conscious presidents."A memoir, to be successful, must be honest," Shenkman says."No president can afford to be truly honest. He can't explain the deals he made, the compromises he accepted, the sacrifices of his principles on the altar of personal ambition. So instead of the truth, we get the president as he would like to be remembered. This is death to a good memoir. There are no revealing anecdotes that explain who he really is or what motivated him. For a person who has spent their life concealing who they are — and all politicians do this to an extent — the memoir is especially unsuited to presidents."

Another problem, historians say, is the speed with which the books are produced....