The exercises were by Betty Edwards, who also posted some before/after examples from her own students on her website... We simply used her book back then, not sure whether you can find some of it also online /on youtube, in case you're interested. It doesn't really help to develop your own style, but it's a good starting point and then you could go for drawing techniques according to your own preference (and not draw like the style Betty Edwards developed in the 70s) after that.