SCHUBERT by George Marek (Viking: $19.95). Contending that "no famous composer has been more falsely represented," Marek strives zealously to dispel notions about Franz Schubert that he was little more than an amiable, plump and bespectacled genius who was uneducated, naive, endlessly poor, given to drinking too much wine, unaware of his own creative worth and completely unacknowledged during his brief life (31 years). To a laudable degree, Marek succeeds in amending such half-truths to form a reasonably clear and more just perception of the composer, one in which Schubert was fully conscious of his own aptitudes, was politically and intellectually alert and was accorded a substantial degree of recognition by musical Vienna. Vividly drawn are the social life, art, environs and political repressiveness of Vienna in the early 19th Century. One also learns much about Schubert and his milieu from telling profiles of prominent members of his wide circle of friends. With partisan affection ("This is not an objective biography"), the author examines many of Schubert's 603 songs and with perception analyzes the failures of Schubert's 15 operas. Marek is less than comfortable, however, in discussing the instrumental legacy.