I recomend you the Jacobson and Shepard Experience.

They have a fanstatic ENG manual.

"Balance Function Assessment and Management.Gary Jacobson, PhD, Neil Shepard, PhD"

Timothy Hain has a great website also.

Finally, the Neuro Kinetics people has several papers on norms, I think.

Every clinic should have its own normative data as there is variation in technique used for VNG recording specially in  caloric test. You should do the test for normal cases and then enter their data into the software of your device and when you test patients the software will calculate the difference between them and normal. Of course reference data from other lab can help but the main reference should be your own lab