I'm learning Differential Geometry (DG) on my own (I need it for robotics). I realized that there are many approaches to DG and one is Cartan's, which is presented in Vargas's book. I think that book is highly opinionated, but I don't know if that's a good or bad thing. Does anyone of you know that book or the approach described in it? More importantly, should I start from this book or should I first learn the popular way (e.g. from Boothby's book)?