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  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?