I was consideriing ordering The 2008 Postal Service Guide to U.S. Stamps, from the USPS. I am primarily interested in the U.S. Airmail stamps, and the defins and commems of the 1970's to the 1990's.
My question is, how different is the older information of those years in say, a 2001 Postal Service Guide to U.S. Stamps? Can I get away with an older one, or should I get the newest edition?

Thoughts?
Thanks!
David