Sigh face. That's not how it works anymore. The Third War for Armageddon - which is 3rd Edition, no longer in print, no longer matters. You can cite it, but claiming that it's anything more than a one-off is hard to prove if you don't have another source.
Priests of Mars, published earlier this year, is considered one of the best non-Heresy books to come out of Black Library in a long time, topped only by Atlas Infernal and Angel of Fire. And, since it's post-4th Edition, it is considered reliable.