I've seen it mentioned already but Lords of Excess is really good. It helped me see Emperors Children in a different light. If you're just in it for the sex, drugs, and rock and roll marines then it might not be what you're looking for. That being said, the Adored have become my favorite EC warband. 

The book depicts them as being prideful and hedonistic perfectionists, like any good EC warband, but they maintain some semblance of nobility. I think it embodies the expression "pride cometh before the fall" really well, which is something I liked a lot about the Fulgrim books. Pride being that gateway to pathological perfectionism resonates with me a lot and it reminds me of that adage in Star Wars, "fear leads to anger, anger leads to hate, and hate leads to suffering". The EC in general, and the Adored specifically, are insecure and afraid of not being perfect. This leads to them being bitter and quick to anger in relatively trivial situations, and this jealousy and insecurity led to their fall to chaos. The legion that could've been the best of them ended up being the most depraved and fractured legion. 

I know I'm droning on but I could talk about this book for hours so I'll leave it with a final thought. The Adored and their leader, Xantine,  have some really nice parallels with the story of Fulgrim and I'd definitely recommend reading it at some point after reading The Palatine Phoenix and Fulgrim. You'll spot the similarities and to me this should be the standard for how EC are depicted. Good luck on your journey and may it be perfect as ever.