The US Solicitor General ranks among the most important, if least appreciated, figures in American law. In this would-be white paper, Caplan (The Insanity Defense and the Trial of John W. Hinckley Jr.) sheds considerable light on the office and many of the men who have held it. His main brief, though, is a stinging attack on what he views as the Reagan Administration's lamentably successful efforts to politicize a traditionally independent post. Required to be ""learned in the law,"" the SG represents the federal government before the Supreme Court, which looks to him for help in choosing cases. With chambers in both the Justice Department and Supreme Court buildings, the SG has influence as well as dual responsibilities that explain without defining his tenth-justice sobriquet. Since its creation during the post-Civil War era, the office has served the nation well, in Caplan's opinion. Former SGs include the likes of William Howard Taft, Thurgood Marshall, Archibald Cox, and Robert Bork. Though noted only in passing, Bork (whose nomination to the Supreme Court had not been announced when Caplan completed his manuscript) comes off rather well; indeed, he's praised for showing ""commitment to the high legal standards of the office."" The current incumbent, Charles Fried, gets appreciably shorter shrift. Caplan not only mocks his appearance and speech but also judges him a ""good-natured mouthpiece"" for White House campaigns (to ban abortion, promote school prayer, gut affirmative action programs, et al.), rather than the legal conscience of government. Other objects of authorial opprobrium are Attorney General Edwin Meese and his deputy, William Bradford Reynolds, whose ideological demands are alleged to have hounded Fried's predecessor, Rex Lee, from office. Whether all pre-Fried SGs were nonpartisan capons resolutely refusing to toe party lines is debatable. Even the sacrosanct ""supreme court,"" according to Mr. Dooley (Finley Peter Dunne), ""follows th' iliction returns."" In the event, without adducing any particularly persuasive evidence, Caplan invites readers to equate ""the rule of law"" with a decidedly liberal agenda--and an activist judiciary. A lively, instructive, but ultimately overstated case which earns at best a Scotch verdict--not proven.