I rarely use my ICBO certification these days, I depend on my SCWI. Why do you want to do special inspection? Lots of work, but $$ is substantially less than what I earn with my SCWI. You also have to buy lots of books for the test:
D1.1
D1.3
D1.4
IBC
AISC-ASD Manual
You must also be proficient at plan reading, you get a full size set of drawings at the exam. You can always tell those not prepared, they rifle through the drawings frantically during the test. There are lot's of places that prepare you for this test. Basically, they sell you the Q + A's.
Did I mention you'll have to pay various jurisdictions their fees to inspect on their turf which equates to more $$$. 1 ICBO license can become 10.