WARNING: Read at your own risk of spoiling the fun. :D ... Seriously, are you positive you want to? I recommend you read first.
"Five more angels swoop down on the one with the snowy angel wings. Judging by their aggressive stances, it's a fight of five against one. Its too dark to see any details on the landing angels but one of them stands out. He's a giant, towering over the rest. There's something about his wings that strike me different. Their wings fold too fast when they land for me to take a good look and I'm left wondering if there is actually was anything different about that one..."
This quote is in the third chapter of the book, quite early on. The description seems insignificant at first, however, it foreshadows what happens towards the end. In this scene, Penryn is hiding under a truck with Paige and her mother. The angel with the snowy wings is Raffe. Oddly, Raffe is an angel who is fighting other angels. When I read this for the first time, I wondered why angels fight each other. Perhaps its comparable to humans fighting themselves, for power. It turns out that among the angels there are political conflicts. If you are familiar with the Bible, you would have figured out that Raffe is an archangel since Raffe is short for "Raphael", one of the seven. Among the archangels, Uriel (referred as "The Politician") desires to be the new Messenger of God, after Gabriel was killed. So Uriel is hunting down all the archangels one by one.
There's a piece of the puzzle that fits within the ending. Notice in the quote Penryn describes one of the "angels" as towering and that his wings are distinct from the other angels. The reason why I said "angel" is because (PLOT TWIST :0) that "angel" is actually a DEMON. Unlike angels, demons have bat-like wings instead of feathery ones. Also, they are typically taller and bulkier than the average angel, I suppose. So, a valid question would be: WHAT THE HECK IS A DEMON FIGHTING ALONGSIDE ANGELS ATTEMPTING TO KILL AN ARCHANGEL? Well, it turns out Uriel, the "bad guy", hired the demon to do his dirty work of slaying Raffe, a threat to Uriel's power. In return, the demon could keep Raffe's wings, which is highly sought due to its beauty.
Well, I spoiled the ending :P . See, don't say I did not warn you. This brief description of Penryn observing Raffe's fight is a valuable clue to angel politics.