As it stands, only way to do campaign currently in the 3025 (Succession Wars) Era is using the Handbook per faction. Admittedly some of the earlier books are dated and don't match up with ATOW, but they can be converted to be usable using the ATOW Companion.
Another era is Era Report: 2750, which gives you another era to play with.
Campaign Operations has conversion charts you can use too, but not for rpg. I'd have look again at that. Combat Manuals (like Combat Manual: Mercenaries) which has a mix of Alpha Strike and Total Warfare information for the 3025 era you could still use. There isn't any ATOW related material in them aside from putting together unit. Their is heavy combat oriented books, but still good source for this era that IS update.