Until retiring in 1990, Lynch ( One Up on Wall Street ) was manager of the spectacularly successful Fidelity Magellan Fund. Here he recalls with self-deprecating humor and disarming candor how he went about choosing winning stocks (and missing a few) for the $12 billion fund, which, during one five-year period in the 1980s, earned investors a 300% return. Lynch strongly favors stocks over other investment vehicles but insists that ``investigative'' research into a corporation's prospects, including credit checks and visits to the firm's installations, is essential. ``Focus on companies, not the stocks,'' he stresses, adding that on this basis limited partnerships, banks and even S & Ls can be sound investments. Lynch's reputation and business writer Rothchild's deft touch should yield big sales for this inside story. Major ad/promo; first serial to Money magazine; BOMC and Fortune Book Club alternates; author tour. (Mar.)