All five of the stories based on a crossover between My Little Pony and X-Com.
Suggested Reading Order:
Stardust
Broken
Fortitude Amicita
Mente Materia
Celestia
Celestia is more of a prequel, but since it might reveal some things I'd suggest reading it last. Broken, Fortitude Amicita are not required for the sequel Mente Materia, but do add to the universe.