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I don't remember too much about this series, but here is what I do remember.
Young man somehow comes across a green demon who becomes his friend and maybe teaches him magic? He shows him these "streams" or "veins" of magic that can be stronger or weaker depending on the place. There are all kinds of different worlds that they can jump to or travel to? they start some kind of business and set up their office in some kind of market on one of the worlds in a tent that is a lot bigger inside then out. The demons name is Azod or Azoth or something like that.
I think there were 7 or 8 books in the series. Any help would be great!