Three to Conquer
by Eric Frank Russell (Avalon, 1956)
Russell selectively breeds two SF storylines — alien body snatchers and telepathic outsider — to produce a memorable invasion tale, told in the manner of hardboiled detective fiction (itself melding lone wolf with FBI manhunt). Cited for possible adaptation during Doctor Who’s planning stages.
Sentinels from Space
by Eric Frank Russell (Bouregy & Curl, 1952)
Even if the intrigue generated by Russell’s shadowy, casually powerful protagonist transpires to be greater than the underlying premise — a conceptual stunner, much alluded to but then minimalist in denouement — the story’s (xeno-)sociopolitical setting alone offers plenty of mileage for intelligent exploration.