After the wondrous, many-headed hydra that was the hyperactive intelligence of First Lady Hillary Rodham Clinton, the serene, smiling Laura Bush seems almost sphinxlike. Washington Post reporter Gerhart, who has covered Mrs. Bush since 2001, has written a judicious, admiring book, more a journalistic meditation than a straight biography, trying to tease out the Texan's ideas and feelings. (There's suspicion, for instance, that she harbors liberal sympathies.) After 188 pages, however, readers may conclude that they don't really care about what makes Mrs. Bush tick. Which is exactly what she wants: Refusing even to take credit for the abstinence from alcohol that redirected George W. Bush's life, she has revealed little of personal interest, other than that she loves The Brothers Karamazov. The sphinx is really a shadow, moving gracefully at the President's heels. Gerhart writes that most presidential biographers devote no more than a chapter to the wives. Mrs. Bush would just smile serenely.