The Watkins Dictionary of Angels: Over 2,000 Entries on Angels & Angelic Beings
This book is wonderful. It's full of useful & wonderful information about angels & the tasks that God has assigned to them.
To this day, I still look things up in this book. I just love it.
The only downside I could find, to this wonderful reference book, is that angels that were part of fiction books, TV shows, or movies are included. So read carefully.