I'd like to nominate The Secret Agent
by Joseph Conrad. It's in the public domain, so free copies in all formats are available all over; it's also available as a recording at LibriVox
. Here's the blurb from LibriVox:
The Secret Agent
The Secret Agent is Conrad’s dark, and darkly comic story of a band of spies, anarchists, agents-provocateurs plotting and counter-plotting in the back streets of London in the early 20th Century. The novel centers on Verloc, a shop-owner, phony-anarchist and double-agent, who becomes embroiled in an ambitious terrorist plan to bomb the Greenwich Observatory.
was ranked the 46th best novel of the 20th century by Modern Library