This book is very reader friendly. It introduces Dynamic Programming, Variational Calculus, Pontryagin Minimum Principle and in its final section some Numerical Methods. Despite being published in the seventies, this is a truly GOOD CLASSIC (there are bad classics).
In my opinion, to the student, Kirk is superior to Citron, to Athans and to Lewis (this is more recent), that is, this book is more concerned about teaching people. Athans is more encyclopedic but much more time-consuming to read.
With Kirk you will really learn the elements of optimal control theory.