In a past thread "100 Books Found in a Strange Library" there was this powerful item that dealt with Favored Enemy.
443. I Hate Everyone But I Hate You the Most
This rather heavy tome seems to be composed of various other books whose pages have been violently erased, mended, and then bound together to create a ugly Frankenstein of a book. It radiates moderate magic and smells of old garbage, worms, and depressing anger.
It was written recently by the famous Ranger Oscar Crouch who was famous due to the fact that every creature/monster/person in the world was his species enemy. The contents are written in small, tight handwriting and each word pressed very deeply into the thick pages.
Any Ranger who reads the entire 200 pages receives the ability to changes his speciens enemy(ies) at the start of each day to whatever he chooses. He must also make a DC 14 Fortitude whenever he uses this abiltity or lose 2 points of Charisma for 24 hours.
That thread has some great books in it.