The Christmas season strikes me as the perfect time of year to recommend Angelology by Danielle Trussoni.
An innocent nun (well, that's sort of redundant, isn't it?) discovers proof that angels once truly existed - and might still. The Rockefellers (yup, those Rockefellers) have hunted these artifacts for decades. Suddenly she's swept into the middle of a war as old as time between the Nephilim, the evil children of angels, and humans. How are we supposed to fight a war most of humanity doesn't even know exists? These aren't your feel good, white-winged Christmas carol angels. They are hard core beings, who've nursed a grudge for centuries until their bitterness colors every decision. (Don't worry - there are still some good angels out there!) This book is full of intrigue, action, suspense, mythology and an epic amount of brilliantly conceived world building. Sort of a National Treasure meets DaVinci code meets Greek mythology. Give it a try!