Most text books on filter theory revert to what I would call the ‘explain by math’ principle. The reader is expected to correctly interpret given equations. The text between equations is often insufficient. This is perhaps ok if you are already acquainted with filter theory, but for the newcomer, a purely mathematical approach can discourage further interest in this subject. Math is needed and most filter guys have learned to embrace relatively complex theories & calculations. It should however be possible to create a good understanding of filters without using a pure math approach. There is of course ‘filter jargon’ and this can be another source of confusion for the beginner. As with every subject there are ‘good books’ and ‘not so good books’. The list below is perhaps an easy start for finding books that suit. Selected classic papers are also a good way for ‘getting into filters’. Among them, the S.B. Cohn paper on direct-coupled resonator bandpass filters or the famous Saal and Ulbrich paper on filter design by synthesis are worth mentioning here.
Articles in technical magazines are another important source of learning material for the advanced level reader. As with books, there are very readable articles among the total amount of articles on filter theory and design. Below are some lists of articles written by important authors.
List of filter design text books
List of publications by Milton Dishal on filter theory & design