Leonid P Lebedev & Michael J Cloud 
CALCULUS OF VARIATIONS & FUNCTIONAL..V12 [PDF ebook] 
With Optimal Control and Applications in Mechanics

This is a book for those who want to understand the main ideas in the theory of optimal problems. It provides a good introduction to classical topics (under the heading of “the calculus of variations”) and more modern topics (under the heading of “optimal control”). It employs the language and terminology of functional analysis to discuss and justify the setup of problems that are of great importance in applications. The book is concise and self-contained, and should be suitable for readers with a standard undergraduate background in engineering mathematics.