Recently I was reintroduced to these two notions through a colloquium lecture, where the speaker briefly explained the concepts. I knew about the envelope of a family of plane curves but never thought of its generalization for surfaces. Due to the beauty of this topic, I tried to study it more closely, but couldn't find a suitable source to do so. Every modern differential geometry book I have consulted has completely ignored these concepts. (I guess the reason is that they have no equivalents in abstract manifolds. Am I right?)
Can anyone direct me to a book or lecture notes or some other source that discusses these concepts in modern language?