1. Vodou is a Haitian system of magic and religion that came about in a merging of African, American, and Western culture, born through the Atlantic Slave Trade, the Haitian Revolution, and a variety of other events. It is an organized religion. Initiation is good to have when working with it and its spirits.
In contrast, Voodoo simply refers to the Louisiana magic that branched off of Haitian Vodou. Gris-Gris, much of the Voodoo paraphernalia we're used to in pop culture comes from it.
The two are related, though not the same.
2. The spirits of the religion could be considered some of the oldest entities on this planet. They are land spirits, and pretty powerful due to their proximity to this Earth. Vodou itself comes from African shamanism, American magic, and Western influences(including Christian)
3. A common element in African belief systems is that there is only one supreme God, who creates the Earth and is so disgusted with what he has made that he abandons it, and the people strive to regain his favor. One such God even vomited the Earth into existence! This creation myth has carried out into Vodou, where the God is Bon Dye(Bon Dieu, being Good God in French). Having more important business, Bon Dye left the Earth in the hands of a group of entities called the Loa who rule the world unrestrained. The Loa/Lwa are essentially the gods that some Vodou followers worship, and the spirits bokors work with. The path itself is fairly primal, in that it focuses on bridging the gap between the human and the loa, bringing the two closer, and creating change. In my experiences, it's one of the best systems for getting things done and getting direct results, but not for ''ascent'', however you may define it.
4. Don't know. I read the Spider and the Green Butterfly:Crossroads of Power by E.A. Koetting but besides a few good things, correspondences, veves, and rituals, it's a very unreliable source and I do not recommend it. My recommendation is to find a way into the current, and learn directly from the spirits.