Sure to be snatched up by defenders as well as detractors of the First Lady, this curiously bifurcated biography is likely to disappoint both groups. While Nancy Reagan's admirers will almost assuredly applaud Leighton's largely sympathetic treatment of her subject's pre-Washington rise to eminence, they will just as probably be incensed by Leighton's resurrecting anew most of the criticisms generated by what many see as Mrs. Reagan's ""paillettes and patronage"" life style. Critics on the other hand will be let down to discover few new revelations about their nemesis's alleged peccadilloes. They will also probably remain unconvinced by the author's speculations about the psychological roots of Mrs. Reagan's controversial personality. By attempting a ""balanced"" portrait, Leighton has produced merely a banal one. The decision to publish this ""first intimate look at the woman married to one of the most popular presidents in history"" at a time when her role in her husband's decision-making processes has yet to be satisfactorily explained is puzzling. It's like issuing a ""final bulletin"" on the Super Bowl game during the third quarter. An ""Epilogue,"" apparently hastily added, attempts to bring the story up to date. The salient facts of the Iran-contra-Donald Regan contretemps (at least as far as they are known at present) are clearly and concisely stated, but the reader is left with a sense of frustration by the lack of either a true conclusion or a knowledgeable prognostication. The writing unfortunately veers from studio-biography gush to gossip-column innuendo. Many of the anecdotes, while familiar, are nonetheless frequently revelatory. Particularly in dealing with her subject's apparent insecurities--the First Lady is more than a little uncomfortable when her husband is obliged to share an elevator with an attractive woman, for example--Leighton adds a welcome three-dimensionality (as well as a plausible explanation of Mrs. Reagan's much-discussed ""protectiveness"") to what is far too often a stereotyped depiction. Most readers, whether ""pro-"" or ""anti-,"" will want to keep looking for the ""Real Nancy Reagan."" Thirty-five black.and-white photographs (not seen).