THE MAGAZINE IN AMERICA by John Tebbel and Mary E. Waller-Zuckerman (Oxford: $35; 448 pp.) . This book is what today's magazines are not: earnest where they are giddy; thorough and staid where they are impressionistic and sensational. Since magazines can be panoramic windows into the colorful coteries of our nation, one cannot help but be disappointed by the authors' gray prose, which portrays magazines merely as business ventures. Still, some headstrong editors do provide splotches of color: We see Whittaker Chambers rewriting foreign correspondents' copy to conform to his extreme right-wing ideology, for example.