Can you help me identify a book I read? It was intended for teenagers. I believe I read it in the late 1980s or early 1990s.
The main character is a teenage human boy from earth, who is learning to use magic with the help of a humanoid lizard companion, who knows all about magic, but can't use magic himself. They travel to numerous worlds.
I remember one of the lessons was to draw in magic from the ground with every step he takes. And there are different lines or streams of magic flowing through the world. Different areas have different concentrations of magic to draw from.
At one point they travel to another world where there is a bazaar or marketplace selling lots of different things.
I believe at one point they travel to yet another world where they are hanged. The main character survives by lifting himself up by using magic, and the lizard companion uses his strong neck muscles to avoid being killed.
I believe there's also a house where the front door is in one world, but the rest of the house is in a different world.