An incredible source for research of Demons ,Devil's, and Djinn beliefs around the works. I picked this up for book research, and it was everything I needed and more. This is a great place to start I'd you're trying to narrow down exactly what continent you want your ghouls to be bases on.
As I like the style of this author, I also had a look at this book. This time she covers the range of demons, devils and djinns with a random selection of evil spirits. Maskim, grimoires, the unbelievable large numbers of demons (what a figure), Arab djinns, Babylonia and Assyria, modern spirits of the Middle East, Chinese Kuei, Japan, India, the demons of the mandrake, Bali. There are even some tales of summoning a damon (I refrain from it) and many impressive impressions. What a great read. Here you find everything about demons you must know before further research. Excellent starting point to become a Demonologist. Highly recommended!