i can only think of 2 things such a book could achieve. answer some of the big unanswered questions such as the true purpose of the exurgent the true reason for all gate linked civilizations being wiped out and the real reason the titans left the solar system/where they went/what they wanted with everybody's head, what the ETIs are really like. i don't think we need to have one answer to any of these questions. hell any explanation of the ETIs nature the reader could understand would pull them down from there position as so advanced/alian trans-humanity cant hope to understand them without a lot more advancement simply because the reader is even less advanced than the trans-humans in EP. the second purpose is providing threats and plot seed's a bit like D&Ds fiend folio. it could describe more variants of exurgent virus, more titan remnant bases and give stats for additional exurgents and titan bots. this could be useful but is far from necessary. it is actually easier to create custom enemies in EP than D&D and you don't need such a wide spectrum of enemies because the power curve is flatter. something that was a threat to your starting character will still be a threat after 20 sessions (at a level every two sessions try sending the 3 orks that nearly killed the party at level one against the same party at level 10). but it would still be useful for isperation and to save time creating enemies of these types, much the same as the NPC file.