IDK, for instance, I'm familiar with Structure and Interpretation of Classical Mechanics, and even though I have had my share of fun from it, the (functional) presentation there still looks terrible, to the point of becoming completely opaque in places, compared to the traditional mathematical notation of the Lagrangian etc. I have also seen books on calculus that focus on function composition (resembling the style of category theory) rather than on sets, and I felt that this was completely unnecessary as it only served to complicate both the exposition and the understanding of the subject that is already difficult enough.