After finding all those epsilon tracts you'll get a special email from marnie that unlocks access to their weird religious scripture on the in game internet, which is actually pretty interesting if you're into the whole cult storyline stuff. The text is basically their manifesto and its honestly pretty funny how it parodies those real world fringe groups, but dont expect any fancy rewards or achievements for your trouble. Good to know for anyone wondering this isn't needed for 100% completion either so you can skip it if you're just going for that.

Just a heads up though, while there's no actual money or vehicles as a reward, the scripture itself is worth reading if you enjoy the game's deeper lore and satirical writing. It adds some nice background to the whole epsilon program mystery, and you might find some interesting connections to other parts of the story scattered throughout their crazy beliefs.