The stories I have read about angels all have the angel helping the main character through a rough patch in their life. For me they are the most relatable as there have been many times in my life where I know I have had my guardian angel watching over me.
Some books that are Team Fallen Angels/Angels:
Hush, Hush & Crescendo by Becca Fitzpatrick
Fallen & Torment by Lauren Kate
Heavenly, Penitence and Absolution by Jennifer Laurens