I read this book after reading Kim because I kept thinking about it when I read Kim. I kept thinking about the different constructions of spying in each of the books. In Kim, spying is a 'game', involves a lot of costuming, knowledge of the habits of the 'enemy' and an ability to move and react quickly to the unfolding events of the 'game'. In The Secret Agent, spying is about indolence, a lot of talk and very little action, strange motivations, and the maintenance of an appearance of domesticity.
This difference is likely the reason why Kim is a successful secret agent, whereas Verloc isn't. Kim only plays the game, and the only personal tie he makes is with the lama, whose own quest parallels Kim's playing of the 'game'. Verloc on the other hand, has 'infiltrated' the anarchists, but it is a group a talkers, not doers. They talk incessantly of the need for revolution, but never take action. In fact, it is when Verloc is pushed into action because his financial spy remuneration is threatened that things go terribly wrong and he ends up murdered, not through the game, but by the wife who he tried to keep in the dark about his secret life. His mistake lies in his attempt to play the spy game while maintaining an ordinary domestic life.
Conrad, as usual, is a writer who entices you into the space of his story, in this case, the space of London (particularly the east end, always the spicer part of town for novelists), and the city is so vividly described that it seems to come alive on the page. I think I like this book even better than Heart of Darkness.