I'm in a seminar class analyzing that book right now, and good god, is it the most hilariously messed up book ever.

Basically, Historia Augusta is best used if you toss aside any aspirations to positivist history (did something "really" happen or not?). Once you look at it as a work of literary satire and as a critique of the late 4th century world, it gets a lot more useful.

Imagine trying to decipher the history of the 80s only through references to that decade in episodes from the 2000s of Family Guy. If you had no other source for the 80s, those references would be completely useless, and speaks more about 2000s era attitudes than 80s attitudes. And yet, if you spend your entire time only trying to enjoy Family Guy for its references to 80s history, you've missed the jokes it was trying to tell about its own time.