Yes it's a classic and very well written. When I read this more than 25 years ago it was a real eye-opener and made the workings of recursion clear to me. Curiously acquiring an understanding of recursion in programming allows me to "think recursively" in regard to many other matters as well, so important for thinking "outside the box".

I'd venture that besides recursion, the other key concepts in programming have been pointers and first-class functions. Getting a grasp on these 3 ideas has been essential to learning.