Lee Harvey Oswald's assassination of President Kennedy in 1963 remains one of the most horrifying and hotly debated crimes in American history. Just as perplexing as the assassination is the assassin himself; the 24-year-old Oswald's hazy background and motivations--and his subsequent murder at the hands of Jack Ruby--make him an intriguing yet frustratingly enigmatic figure. Because Oswald briefly defected to the Soviet Union, some historians allege he was a Soviet agent. But as Peter Savodnik shows in The Interloper, Oswald's time in the U.S.S.R. reveals a stranger, more chilling...