When MGM studio chief L.B. Mayer got really angry, he'd hurl himself to the floor and roll around on the rug. (For extra laughs, imagine a full-grown brat who looked, according to biographer Scott Eyman, ''like a very small, very charming white penguin.'') One director cracked that the ''L.B.'' stood for ''Lousy Bastard.'' Yet Louis B. Mayer was also a self-made movie genius who essentially invented the star system (he discovered Greta Garbo on a visit to Berlin) and, over 27 years, led MGM on the greatest run a studio has ever known. While Lion of Hollywood might lack ''Golden Age'' polish, Eyman's epic, 516-page bio builds adeptly on Mayer's legend, casting him, in a fair but tender accounting, as trickier and more complex than most of the glossy films he made.