Eric Frank Russell
Wasp
Despite being very obviously dated (the “aliens” in the book talk and act exactly like New Yorkers in the 1940s, and the agents of the evil alien government act exactly like Nazis), the book itself is still quite a bit of fun. It depicts the efforts of a single disguised human agent to destabilize an entire alien planet in preparation for an invasion. Anachronisms aside, this is a swift-moving novel that holds your attention, and if you’re looking for something short, old-fashioned and engrossing, this admirably fits the bill.