I am reading now E. Soper, Classical Theory Of Fields, now and sometimes it is very hard to follow the equations. So I need a side book on classical field theory to read it comfortably. Landau & Lifshitz's book is not helping as its content and topics are very much different.
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The classical reference is Landau & Lifshitz, The Classical Theory of Fields, from the Course of Theoretical Physics. As all Landau & Lifshitz books, masterpieces [in my opinion] full of content but sometimes a little difficult to grasp for beginners.
Along with Landau and Lifshitz, there are books which although not explicitly about classical field theory, have good treatments. Chapters 11 and 12 of Jackson's Classical Electrodynamics are about special relativity and field theory, and I would recommend Goldstein's Classical Mechanics as an introduction, where field theory is introduced in some of the later chapters.