Dr. Christopher died in 1983...so if the book was actually published in 1976, then it would be an original unedited version.
HOWEVER, since it indicates 2001/13th edition, I'd say it's VERY likely to have been edited. If it's the 25th Anniversary Edition, it's definitely edited :(
Blessings -
Unyquity
P.S. I have the 25th Anniversary (edited) edition, and it still contains VAST amounts of awesome/correct information. It's worth the money even if it has been edited. If you're interested in a CD copy, PM me, and I can point you in the right direction.