<p>A GROANING SHELF of evidence bears out what many people know intuitively: The American mass media suffer from a left-wing slant. The data come in a variety of forms: classic studies such as ''The Media Elite'' (first published in 1986) and William McGowan's ''Coloring the News'' (2001), insider exposes like CBS veteran Bernard Golberg's recent bestseller ''Bias,'' and a thick sheaf of industry studies and public opinion polls.</p>