So, I've officially started studying for my CCIE. I downloaded the CCIE exam topics matrix from the Cisco site which contains some valuable resources for studying assistance. Besides that I have a list of books I'll be using, various training videos, and so on. The first topic involves the difference between IOS and IOS XE. There are three pages in the CCIE OCG book, which lightly touches on the subject. I found a Cisco Press book which devotes a whole chapter to the differences between IOS, IOS XE, and IOS XR. I figure that would probably have more than enough to cover what I need to know for the test.