Sam Houston Johnson shoots straight, from the hip, and Lyndon Baines has been holding his breath. There's plenty of plain talk that'll stir up a storm, on the Pedernales, but Sam (or his editor) has stopped short of irreparable damage. In fact, the blunt portraiture by this lifelong Big Brother-watcher has an undeniable scrappy charm; his flesh-and-blood LBJ is far more appealing than the poker-faced President of solemn TV speeches (Sam always warned him against that medium). This Lyndon certainly has his shortcomings of temper and temperament--""small wonder that he has often been surrounded by scared-ass sycophants""--but he was the greatest legislative strategist of this century, and sam sure has it in for the newspapermen and New Frontiersmen who gave him short shrift. Private enemy number one is Bobby Kennedy, though Sam confesses he was a real pro in politics, where ""ambition and egotism are the twin names of the game,"" Along with the political snipes, there are personal touches: LBJ constantly scolding Sam for falling asleep with the light on, taking out his Presidential tensions in a brotherly domino game, assigning Sam a Secret Service agent (under code name 007) to keep him on the up-and-up. Silent Sam's finally had his say and he will be strongly promoted: