Gerson's assumption that the reader will have no previous knowledge of Walker or Latin American history is the only tip-off that this biography is intended for a juvenile audience. But even this author's journeyman talents (most noticeable when historical controversies are decided on the basis of preconceived notions of character) can't dim the appeal of William Walker, the epitome of the filibuster in the word's other sense of a ""military adventurer."" After failing to take Sonora (later the Gadsden purchase) from Mexico, Walker recruited his own private army to intervene on the side of the Liberals in Nicaragua and became that country's most powerful personage and, briefly, chief executive. It seems that Walker was defeated, not by his imperialistic ambitions, but by his unworldly idealism which brought him into conflict with the Central American interests of Commodore Vanderbilt and, later, the British. This popular presentation which comes down heavy on the side of romantic pathos is obviously not the last word on Walker's quixotic adventures, but its tone suits Walker's self-created legend and you have to credit Gerson for coming up with a subject who can't fail to fascinate.