Well let me see if I can help. The different planes, are like layers. Except, when you get to the higher planes, you can see through spirits guises. Magicians can only see planes 1-3, and slightly on 4 with their lenses. The higher level spirit, the more planes they can see. So, for example, Simpkin(Sholto Pinn's foliot), can only see up to planes 4(I think, maybe it's 5). But then Bartimaeus can see up to 7. Then spirits higher than Bart, can see even further than 7. So, how things get hid on them. The spirits can change their apperance on the lower planes. But, when you look on the higher planes, you can see their true form.
I hope this made sense! Let us know if you're still confused.
The Mona Lisa:
