A Primer on Pontryagin's Principle in Optimal Control: Second Edition by I.M. Ross. https://www.amazon.com/Primer-Pontryagins-Principle-Optimal-Control/dp/0984

A Primer on Pontryagin's Principle in Optimal Control: Second Edition by I.M. Ross. https://www.amazon.com/Primer-Pontryagins-Principle-Optimal-Control/dp/0984357114 https://www.amazon.com/Primer-Pontryagins-Principle-Optimal-... Explains optimal control theory and the pontryagin minimum principle from a geometrical perspective rather than the confusing derivation from calculus of variations that is more typical. Also explains it at an "Engineering" or applied math level rather than at a mathematical foundations level so the results are more usable. [ I'd describe this book as the "Div, Grad Curl and All That" of Optimal Control Theory if that makes sense to you ]