I am interested in knowing if there are any introductory textbooks on relativistic non-instantaneous action-at-a-distance interactions. The abstracts that I've read on this topic recently   seem very interesting and I would like to learn more. I have also read that Henri Poincaré proposed two laws of gravitational attraction compatible with the principle of relativity in July, 1905 http://www.univ-nancy2.fr/DepPhilo/walter/papers/b4.pdf and I'm particularly interested in knowing what he was up to. I'm looking for a detailed and easy to understand mathematical explanation of his work. Any suggestions?