Lecture notes by Hector Sussmann
300 years of optimal control, a historical article by Hector Sussmann and Jan Willems
Feynman's lecture on the principle of least action
Lecture notes by Alberto Bressan (scroll to the bottom of the page)
Tutorial on viscosity solutions of HJB equation by Alberto Bressan
Contributions to the theory of optimal control, 1960 paper by Kalman
An introduction to the coordinate-free maximum principle, paper by Hector Sussmann about maximum principle on manifolds
Contact geometry and the maximum principle, paper by Tomoki Ohsawa