Tolkien, one of his generation's most respected linguists, was ''earmarked'' to crack Nazi codes in the event that Germany declared war. Intelligence chiefs singled him and a 'cadre' of other intellectuals to work at Bletchley Park, the codebreaking centre in Buckinghamshire. Its staff - which included Alan Turing, the gay codebreaker - would later decipher the 'impenetrable' Enigma machines. This saved Britain from German conquest by allowing the Navy to intercept and destroy Hitler's U-Boats. According to previously unseen records, Tolkien trained with the top-secret Government Code and Cypher School (GCCS). He spent three days at their London HQ in...