I have this book and think it's great. It covers Linux 2.6, which happens to be the version of the kernel used in CentOS6.2 which is what I use at work. What are the major differences between 2.6 and the 3 line. Specifically, what is the major difference that caused the numbering scheme to go from 2 to 3?

The jump to 3 was made arbitrarily after 2.6.39. Here's the announcement last summer from Linus: https://lwn.net/Articles/452531/

Basically, Linus just felt that the minor release numbers were getting too big. He dedided to not call that release 2.6.40 and call it 3.0.