Timothy Zahn 
Quadrail Series Books 4-5 
The Domino Pattern and Judgment at Proteus

Action science fiction, mystery, and espionage combine in books 4 and 5 of the Quadrail series from this Hugo Award–winning author.   The alien Chahwyn created the Spiders to keep their intragalactic transportation system, the Quadrail, safe and running smoothly. And ex-government agent Frank Compton is there to protect its integrity. So far, Compton and his beautiful half-human, half-Chahwyn partner, Bayta, have had their hands full keeping the Quadrail open and preventing the evil Modhri from using it to spread their mind-controlling infection. But new threats are always popping up . . .  The Domino Pattern: While the Quadrail is en route to the farthest edge of the galaxy, someone is poisoning passengers. Meanwhile, Compton and Bayta must prevent the system from derailing, and unless they can unmask a sinister conspiracy, the ordered universe will fall into chaos.  Judgment at Proteus: The Shonkla-raa are invincible fighters dedicated to the destruction of the Quadrail. They were once thought to be extinct, but no longer. Now Compton and Bayta must join forces with their most feared enemy, the Modhri, if they hope to protect all species along the Quadrail from annihilation.