Demonology

One place to start is with the Lesser and Greater Key of Solomon. You can read those for free on-line athttp://www.hermetics.org/library/Library_Grimoires.html

There may not be one best book on demonology, as there are so many differing beliefs of demons and types of spirits throughout the world, even in the Western tradition.

For example, some people study the Goetic spirits in depth, and call themselves deomonlogists. They, are, after all, studying demons. The same, a person who works with entities related to a specific branch of Luciferianism might make the same claim. Neither is wrong; both are right. Demonology can be found among many traditions across the world.

I believe that 'demons' aren't all evil, and not all angels are good. There's a demon that protects me, and I've never done a pact with him, so I think he's just my guardian, or maybe my spirit guide.