There are 72 demons listed in the Ars Goetia, which is the first section of The Lesser Key of Solomon, a grimoire that provides instructions on how to summon and work with these demons.

Depends on what your interest is: Summoning them? There's a reason the Goetia is a classic. However, it is extremely complex. But as a general rule, you don't want the "Summoning infernal entities for Dummies"

If its just information on various Demons/Demonic entities, there are a variety of dictionaries available depending on if you are looking for Judeo-Christian or other religions.

Beelzebub or Baelzebub or any other way you chose to spell it, is A demon, though I'm not sure entirely what you mean by "the" demon. From what historians can put together, this particular creature (first listed in the Goetia, the Lesser keys of Solomon, a supposedly 12th century Grimmoire derived from the practices of the famed wise Solomon the king) seems to be a corruption of Bael (or Baal)- Zebul, a Mesopotamian sky god, into Beelzebub, meaning 'lord of the dung heap' or 'lord of the flies' depending on how literal your interpretation.

It was ruled by the God of the Dead named Hades. The Underworld was a dark and gloomy place. There were the Fields of Punshment, Elysian Fields, and many more. The Fields of Punishment were for the people who did BAD things in their life time. Elysian Fields is where all the Happy people and warriors would go if they lived a good life.

There isn't one. The "key of Solomon" has always been symbolic, regardless of its context.

The earliest mention of this key are the "clavicles of Solomon" 16-17th century grimmoires (the lemegeton and goetia and I believe there is a third..) that taught how to summon angels and demons to one's bidding, supposedly based off the hidden teachings of King Solomon from the bible.

The most prominent use of the term "key of Solomon" refers to free masonry. Since the masons teach ethical and moral ideas through the myth of Hiram Abif (the architect supposedly responsible for the construction of the temple of King Solomon), the key of Solomon is the understanding that masonry gives to its practitioners about the universe and god.