I think that I am all alone in thinking that this book is one of the greatest books written, but none-the-less it is my opinion. I think it is the best of Conrad's work. It certainly has his eccentric writing style on display, but I never had a problem with that.
Often characterized as a spy novel. It is really the study of a marriage. Mrs Velloc is one of the great characters in fiction, and the climatic scene of the novel one of the best their is. Its really a novel about self-sacrifice and the perils of self-sacrifice. The spying and terrorism is what makes the plot go.
There is a lot of subtlety to the portrayals here. I don't think that people catch on to what Conrad is trying to do. Until one realizes that the main character is not Velloc, but Mrs Velloc you can't undrstand the book. Mr Velloc is a great character as well, because he is a lovely study in evil, and Conrad nails him right on the head. But that climactic scene might be my favorite scene in any work.