Those plays were not written by Shakespeare; They were penned by another man with the same name. Sheriff Pat Garrett did not gun down Billy the Kid. Billy died in an old folks home in Tulsa, Okla., in 1939. And that gaunt ancient held in Berlin's Spandau prison for over 30 years is not Rudolf Hess. Of course not. He is Hess' double. For this latest amendment to history, we are indebted to a British army physician, W. Hugh Thomas.

Rudolf Hess was the Hitler deputy who in May of 1941 stunned the world by parachuting into Great Britain to try to talk British leaders into a separate peace with Germany. The British said no, than you, and Prime Minister Churchill had Hess clapped into prison for the rest of the war. Later, he was sentenced by the Nuremberg war crimes tribunal to life imprisonment and transferred to the prison fortress at Spandau. There Hess has remained, the only remaining prisoner in a huge prison operated on a monthly rotational basis by the United States, the Soviet Union, Great Britain and France. And now we learn that this extraordinary apparatus is caging an impostor.

How did Dr. Thomas make his discovery? In October 1972 he was posted to Berlin as consultant in general surgery to the British Military Hospital. Thomas was present when Hess was being examined by other doctors for a gastric complaint, and thus briefly Thomas saw the famous prisoner stripped. By Jove, he spotted something truly extraordinary! Where was the scar from Hess' old war wound? Thomas had done some reading on Hess and had learned that the old Nazi had been shot in the left lung during World War I. The missing scar stirred Thomas' curiosity, and a few years later he began poking into the background of the Hess case.

From his research he concluded that Rudolf Hess did take off in a Messerschmitt and probably did head for Scotland on his one-man peace mission. But he never made it. He was shot down over the North Sea by German fighters. A double was then substituted for Hess, who flew another Messerschmitt to Scotland where he passed himself off to the British as the real Hess.

Conspiracy theories are fun, much more satisfying than the tedium of logic. And this book is a delight. Dr. Thomas writes cleanly and sanely, anticipating and responding to every doubt. His book allows the mystery-loving amateur to match wits with the author in a diverting, but not particularly difficult, intellectual puzzle.

Not too difficult, because the author asks the reader to accept that within two or three hours after Hess' secret take-off, a perfect look-alike, who also happened to be a crack pilot, departed from another airfield and completed Hess' flight. And, all of this was done without Hitler's knowing, for as Thomas himself admits, the Fuehrer almost went into shock when he learned of his faithful deputy's mad maneuver.

Further, we have to believe that over the next 35 years, the fake Hess managed to fool British officials (some of whom had met Hess before the war), Nazi cronies, with whom he was tried and incarcerated (including the super-sharp Albert Speer) and finally, his wife and other family members.

Why would certain Nazis want to send a Hess double to England to carry out the same mission that Hess himself set out to do? The power behind the plot, Thomas believes, was Heinrich Himmler, that fearsome policeman of Nazi Germany. Himmler had been scheming all along to bounce Hitler, take over the Reich for himslef and make peace with Britain. Hess was a rival. When Himmler learned that Hess was moving ahead of him on the British peace gambit, he had Hess shot down. And then he re-created the rival he had just destroyed by putting in a double to play Hess? Come on now, Dr. Thomas.