The Secret Agent [suggested by Richard Frost]

Behind the façade of a Soho shop selling dubious merchandise lives Verloc with his wife Winnie, her mother and her retarded brother, Stevie. Verloc is an overweight indolent anarchist who conceals his political activities, such as they are, under a veneer of domesticity and family life. [Product description from Amazon]
The novel has its own Wikipedia entry and has been made into a film. It is also available to read online.
About the Author
Wikipedia page.
“On-line Literature” entry.
Fan society site.
Shortlisted for this month
Richard’s other two choices for this month were:
The House of the Spirits

Spanning four generations, Isabel Allende’s magnificent family saga is populated by a memorable, often eccentric cast of characters. Together, men and women, spirits, the forces of nature, and of history, converge in an unforgettable, wholly absorbing and brilliantly realised novel that is as richly entertaining as it is a masterpiece of modern literature. [Product description from Amazon]
The book has its own Wikipedia page and has been made into a film which also has its own Wikipedia page.
About the Author
Wikipedia page.
Author’s website (English version – it is also available in Spanish).
The Master and Margarita

I am not going to attempt to include a synopsis for this novel, which has devoted fans the world over. The extent of its popularity can be judged from the fact that not only does it have a Wikipedia page it also has an entire website devoted to it (which has its own Facebook page), and it has been made into both a film AND a TV mini-series.
About the Author