Difference equations applied to DSP
https://ccrma.stanford.edu/~jos/filters/ and specifically
https://ccrma.stanford.edu/~jos/fp/Difference_Equation_I.html 
http://www.dspguide.com/pdfbook.htm
https://www.amazon.com/Digital-Signal-Processing-Alan-Oppenheim/dp/0132146355
    (need to buy the book)