LOU CANNON's Reagan is the sort of well-crafted biography of our incumbent president one would expect from a man who is political journalism's senior Reagan-watcher and who holds down the position of White House correspondent for the capital's most powerful newspaper. On one hand, reporter Cannon has tapped his long acquaintance with Ronald Reagan, his memory, his files, his relationships and his insider access to come up with a thorough and readable portrait of the president's life and political odyssey. On the other hand, inasmuch as Cannon presumably wishes to go on Reagan-watching for another 2 1/2 amply-accessed years, he has written little -- by way of analysis, anecdote or personal revelation -- to provoke an angry hail of jelly beans when next he sets foot in the Oval Office.

Does this lack of critical Reaganalysis detract from the book? By reasonable criteria for a biography of a sitting president, probably not. Besides, the library of U.S. politics has so far lacked a broad, detailed life of Ronald Reagan up through his presidential accession by a sympathetic yet detached insider. Cannon's volume fills that role -- and should remain preeminent for the foreseeable future. And had Cannon displayed a penchant for critical probing, he might have sacrificed the above-the-combat niche his book seems sure to enjoy. If you want to know the fullest available detail on Reagan's early- 1930s engagement to college sweetheart Mugs Cleaver, Cannon has it. If you want substantial background detail on Reagan in Des Moines in 1936, in Hollywood in 1950, in Sacramento in 1967, at the Republican convention in Detroit in 1980 or in the White House in 1981, it's there.

The flow is effective, though. Lou Cannon's prose will not stand the competition of monokinis for jaded White House aides planning to read it on the beach at St. Tropez. But for the average reader spared such distractions, the book moves right along. Ronald Reagan's life has been an interesting one, and the political part of his career kind of sneaks up on you, in contrast to the almost wholly political lives of Lyndon Johnson and Richard Nixon.

Persons themselves less than wholly committed to politics will probaby find this a plus. In contrast to his predecessors, Ronald Reagan became a public servant rather late in life. Aside from his World War II military service, Reagan did not clamber onto the public payroll until after reaching his mid-fifties, the first president to come to government service so late in life since Woodrow Wilson.

Cannon does make some effort to explain how Reagan's early life in the Midwest and in the movie business shaped his later views, but not much. That's unfortunate. It would have been intriguing, for example, to see Cannon's (presumably differing) explanation of the impact of what Jules Feiffer calls "Movie America" in shaping the president's thinking. By Feiffer's thesis, Reagan is in some measure still a prisoner of the image of America that Hollywood sought to purvey in Reagan's time. Thus, the administration, like the moviemakers of yore, has been trying to recapture an America of white picket fences, church suppers and Norman Rockwell culture. That seems an exaggeration, but what role did Hollywood play in the shaping of the presidency, 1981- 84? Woodrow Wilson's cloistered past as a professor and university president bespoke more than a few of the weaknesses that would soon be apparent in the Wilson presidency. In the case of any president who turned to public servic in his fifties, one would think that a rigorous examination of the legacy of his prior occupations would be in order.

There is not much of this in Cannon's book, perhaps because with only one conspicuous exception, the author eschews any broader historical value judgment. Relatively few paragraphs in the first four-fifths of Reagan even hint whether we are dealing with a successful or unsuccessful president. And that, too, may be wise. History's verdict is never in by a president's second year in office. The cautious biographer of a sitting president will merely pave the way for later overviews rather than attempt to render them prematurely. Committed skeptics already looking for that verdict will find the great bulk of Reagan something of a literary Chinese dinner -- filling, but not especially memorable.

In a related vein, White House aficionados know that important political figures, on receiving a new book, do not turn to the table of contents but to the index -- to look up their own names (William F. Buckley, a connoisseur of such proclivities, once sent a friend a book inscribed "hi!" at the appropriate place in the index). And in this regard, the present semi-fratricidal quartet of senior White House hierarchs -- Messrs. Baker, Meese, Clark and Deaver -- need have no fears. Baker, Clark and Deaver come off very well, and nothing is done to Meese in Cannon's book that has not already largely been achieved by the operation of time, politics and Meese's managerial shortcomings. Cannon is not a man for either denigrations or Machiavellian counterpoints.

If I have one major criticism of Cannon's work even in his chosen vein as cautious biographer of a sitting president, it is the partial volte-face in his second-to-last chapter on "The Delegated Presidency" and his short four-page epilogue. In both sections, Cannon begins to cast serious aspersions on Ronald Reagan's intellectual energy and depth of policy comprehension -- "His ignorance was his armor, shielding him from harsh realities which might have discouraged some of his boldest initiatives while gradually weighing down his presidency. He did not know how the federal budget worked or understand the threat to the environment from toxic wastes and acid rain or realize that the Soviets, for all their menace to human freedom, had in fact fulfilled a number of their treaties." In the epilogue, we are told that Reagan did not really understand the economic program he offered, and that in 1982, the president "became noticeably more stubborn and isolated." And then, in his last 500 words, we are told that Reagan is ''running out of time. His reach had exceeded his grasp. His knowledge had proved unequal to his courage." The author also proclaims "I believe that Reagan will not run again."

For better or worse, these last chapters do not really go with the book. It is tempting to characterize them as a belated attempt to tack a historical judgment onto a book which, through most of its pages, sought to avoid that commitment. To some extent, the judgmental aspect of the concluding chapters and epilogue serves to spotlight the omission of the rest of the volume: Too little explanation of just what about Ronald Reagan's origins, upbringing, Hollywood background and political life made him what he is -- or isn't -- in the White House. My own sense is that Cannon's book lays insufficient groundwork for explaining the failure the author himself begins to proclaim late in the book, a clear structural deficiency at one end or the other.

Too say this is not to suggest that a Reagan biographer -- Cannon or anyone else -- should try to apply the assorted psycho-pathological yardsticks (from early toilet-training to mother fixations) used on past chief executives like Richard Nixon and Jimmy Carter. The ''presidential character" of the Great Communicator can safely be left to Professor James Barber et al. Yet as the presidential fortunes of Ronald Reagan become clearer, the role of his own outlook and background in his success or in his failure or in some amalgam of the two will gain importance. Lou Cannon has written what should be the definitive pre-1984 biography of sitting President Reagan. But his inadequate, final-stage flirtation with rendering a historical judgment only underscores the larger extent to which that role, at least, has been left for a future chronicler.