"The Secret Agent" by Joseph Conrad

Later in his career, Joseph Conrad's novels became more political, and "The Secret Agent" is one such example. This is about a spy for an unnamed country (presumably Russia) stationed in London in 1886. Possibly because of its focus on terrorism, among other things, it did not sell well in the U.S. and the U.K. upon its publication in 1907, but has since undergone a re-evaluation and now is considered one of Conrad's finest novels. Fun fact: it apparently was a big influence on Ted "Unabomber" Kaczynski.