The Spy and the Traitor by Ben Macintyre is an engrossing work of nonfiction subtitled The Greatest Espionage Story of the Cold War. It's about Soviet KGB officer Oleg Gordievsky who spied for the British government and the book reads like fiction it has such remarkable events written of.
Gordievsky came from a KGB family and in his early twenties was in Germany as the Berlin Wall started to be built and he disapproved of the crackdown on freedoms for citizens in East Germany. He then became a KGB spy, drawn not to the ideology of the Soviet system, which he felt could change for the better, but rather the allure and glamour of intelligence work. He started in the Soviet embassy in Denmark in 1966 and was part of 20 officials there, with 6 of them actual diplomats, and 14 working for either the KGB or GRU, Soviet military intelligence. Gordievsky worked with the patchy network of illegals in the country and was disgusted by the Soviets sending troops into Czechoslovakia in 1968. He began spying for MI6 based on this disillusionment with his government and it was interesting reading how much of the information that he gave to the British then had to be altered to conceal its source and parsed out slowly and in drips to the groups that would benefit from having it.
After his time in Denmark came to a planned end, Gordievsky was sent back to Moscow for three years and had no contact with MI6 but decided to learn English. He then was posted to the Soviet embassy in London and resumed passing along secrets to the British. One of the more astounding ones was that the KGB genuinely believed the United States would launch a pre-emptive nuclear strike, and that Soviet leadership thought the NATO war game ABLE ARCHER in 1983 was the start of WWIII. Information such as this, which was passed along to the United States with Gordievsky’s identity concealed from them, about Soviet paranoia helped lead to a slightly different approach from the West, and more of a thaw in relations. He also gave tips on how Margaret Thatcher should act at the 1984 funeral of Yuri Andropov, and later about how to best interact with new Soviet leader Mikhail Gorbachev.
Gordievsky was close to becoming the rezident, or lead KGB officer in the London Station, but came under suspicion of being a spy for the West, likely from mention of a mole being made to the Soviets by U.S. Intelligence Agent Aldrich Ames. Gordievsky in 1985 was called back to Moscow for meetings and interrogated by the KGB as they knew there was a mole somewhere, quite possibly in London, but didn't know who it was. There was a lot of circumstantial evidence of his guilt, but no proof. The KGB interrogated Gordievsky and tried to get a confession from him, with perhaps him being saved by his vehement and angry denial of guilt. Even if the KGB was 99% certain he was a spy, they didn't want to get in trouble on the off chance that he turned out to be innocent. This same principle helped Gordievsky when he attempted to escape from Moscow, via operation Pimlico by the MI6 exfiltration team. He shook his KGB followers but they didn't want to report losing him, rather hoped they'd find him again and avoid getting in trouble. It was an amazing story of the effort by he and the British to try to get him to safety, capping off an engrossing book.