Wilhelm Klingenberg 
Closed Geodesics on Riemannian Manifolds [PDF ebook] 

This book contains expository lectures from the CBMS Regional Conference held at the University of Florida, 1982. The author considers a space formed by all closed curves in which the closed geodesics are characterized as the critical points of a functional, an idea going back to Morse. This exposition gives a refined version of Morse’s approach which has several advantages over the old one-in particular, it possesses a canonical $/mathbf O(2)$-action.