In the far-flung future, Earth's citizens have left its depleted shores for space, and those staying behind have, over the centuries, devolved back to a Bronze Age state. Then, suddenly, an alien invasion arrives to pick the planet's bones in Avram Davidson's genre-bending The Kar-Chee Reign.
I have credited Davidson for my sudden interest in what I used to call "hippie-fi" after years of reading about lantern-jawed space heroes via Heinlein, Asimov, and those type of writers. From Davidson I decided I would take on Philip K. Dick, John Brunner, Samuel R. Delany, and others, and I haven't looked back.
The book that set me on that fateful journey was Rogue Dragon, and I was pleasantly surprised to find out this book was the prequel. In it, a group of makeshift heroes, proficient with bow and club and rousing speeches, outsmart the alien science of the insectoid invaders.
Avram Davidson is a solid storyteller, and used some interesting socio-political underpinnings to prop up his story.
I got this book from http://www.paperbackswap.com/, an Ace Double which has Ursula LeGuin's very fine, similiarly-themed Rocannon's World (which I recently finished) on the flip. It may be the best combined Ace Double I've read.