My textbook says two different things and I'm not sure how to reconcile these two:
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electric field inside a conductor is always 0.
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for a conductor with a cavity with a charge q inside it, the field due to the charge q for all exterior points is cancelled by the induced charge on the inner surface.
The charge q will induce charge of -q on the inner surface, and because the conductor is neutral, there will be charge q on the outer surface of the conductor (leftover charge). Due to (2), for all exterior points the field due to q and charges present on the inner surface is cancelled. However, the field due to the induced/leftover charge on the outer surface isn't, so there will be some field due to this charge inside the conductor, which contradicts (1).
Best Answer
The point is the charges on the outside reorganize themselves so the net field is $0$ inside the conductor. The charge distributions on the inside and outside surfaces need not be constant and in general will be quite messy unless the geometry is simple.
In the example below of a source charge off centre inside a hollow sphere, notice how the positive charges on the inside surface are not uniformly distributed, but how they are uniformly distributed on the outside surface.
The net result is that the field outside the cavity only depends on the distribution of the outside charges, but the field inside the conductor is still $0$ because of the skew charge distribution on the inside surface.
[Image source: Haliday Resnick Walker 10th edition]