Alien invaders tend to shoot acid, go invisible, or drive humongous ships. Not the ones in Rudy Rucker's 1980s classic The Sex Sphere, where an alien named Babs and her crew take the form of disembodied sex organs that attach to human hosts. Once attached, the humans go into orgasm pleasure dreamland, and never really do much again. How to stop such a clever attack? Our hero must battle the biggest vagina you've ever seen — a vagina from beyond space-time. Only trip-tastic writer Rucker, author of Postsingular, could imagine such a scenario.
The best part is that Rucker, a mathematics professor, opens the book with a whole introduction on the fourth dimension and how it works. The aliens, you see, are trying to return to this dimension and so the reader needs a bit of grounding in theoretical physics before delving into the sex. It's quite possible that Rucker designed the entire book to seduce horny teenagers into learning some math.
If you like your science fiction to contain hard science mixed with bizarro humor, don't miss The Sex Sphere. Painting by Rudy Rucker.