In the Bartimaeus Universe, it is regularly reported that the earthly plane consists of 7+ seperate but overlapping planes, on which varying levels of magical activity can be viewed. According to Bartimaeus, the more powerful a spirit is, the more planes they can typically view. In terms of human kind, most can only see the first and sometimes the second under certain circumstances, while those with certain kinds of magical resilience can see 1-7. Magicians often wear lenses that allow them to see the second plane and the third, and a little bit on the fourth.
Lower level spirits, such as imps and foliots, can typically see along the lines of 1-5 planes, meaning that anything that has the same appearance on those planes but not the higher ones will be able to hide their true shape from them.
Cats can see 2 planes.
Higher classes of being, such as djinn, afrits, and marids, can see 1-7, with some claiming to have access to even more planes. (Bartimaeus views these spirits as show offs, as only planes 1-7 are considered important).
Existence of other planes is briefly hinted at. Bartimaeus believed that the talismans called the Seven-League Boots were activated and operated via the imprisonment of one or more spirits that had access to a theoretical "8th plane", which allowed the boots wearer to travel extremely long distances in very short periods of time. He noted that he knew little of such things and spoke no further of matter, never again mentioning it in any story.