Do spirits level up?
For example, does a djinn (Bartimaeus) start out as a level 4 djinn and is that way forever? Or does he/she start out at a level 1 djinn, then proceeds to level up? If so, is the speed of which they level up determined by the amount of summoning or something else?

Their power is fixed (less weakness from being on Earth too long in a single stretch) and depends on the strength of the spell that originally created them. They don't gain extra power, though they certainly get smarter and better at using the strength they have.. if they survive long enough.

Thank you very much :)