I intend to approach the paper of Wolfgang Ziller: "The Free Loop Space of Globally Symmetric Spaces", but I need the proper background on the foundations of the study of Free Loop Spaces. I obtained from my library the "Lectures on Closed Geodesics" by Klingenberg, but found it to be rather dry and difficult to read. Do you have any other suggestions?
This survey paper on the Morse theory and closed geodesics: http://arxiv.org/abs/1406.3107 (Morse theory, closed geodesics, and the homology of free loop spaces, by Alexandru Oancea) provides a lot of relevant references. In particular, it says that "a beautiful reference concerning closed geodesics is the book by Besse": http://link.springer.com/book/10.1007/978-3-642-61876-5 (Manifolds all of whose Geodesics are Closed).