The Spirit World is made up of ghosts and spirits that do not reside within a body. There are ways to contact people from the spirit world such as using an exorism or talking boards. Some certain people such as ka servants or mediums act as the bridges between the spirit world and earth. There are souls that visit earth because they do not have peace within their death, or they have something else to do.
During the beginning of the fourth novel, there was a earthquake in the spirit world where it shook most violently and hauntings began around the Temple of Dendur. Many of the spirits, however, disappeared. Faustina Sachertorte was almost pulled in herself if not for escaping at the last moment. This was later revealed to be caused by Iblis sucking up spirits through the terracota warriors to power his Jade pyramid.
Despite their gifts, djinn are unable to use their powers in the spirit world and are almost dependent on the power of their own spirits when they enter.