Volume I has the detailed descriptions of the Diagnostic Trouble Codes (P1135, P0420, etc.) and the detailed troubleshooting procedures you use to determine where the failure is that caused the code. That's 80% of the book and it is pretty essential information. The engine sections in Vol. II are what you use to repair or replace the failed parts now that you know what is wrong. Also essential. I use both regularly. I bought them in '99 before there was any hope of seeing them on-line, but I am glad have the exact book for my year. There is a lot that is different about the '02 on-line from my paper '99.
I did NOT buy the wiring manual, and I haven't really missed it. I have been able to use the on-line one when I have needed to research something.