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i always found that working thru examples with Schaums, helps to clarify the points i was not understanding.
Refreshing general theory => Schaums Outline of Digital Signal Processing
https://www.amazon.com/Schaums-Outline-Digital-Processing-Outlines/dp/0071635092
If you have Matlab access, you might want to also look at DSP w/MATLAB.
https://www.amazon.com/Digital-Signal-Processing-Using-Matlab/dp/9386668114
Schaum's Outlines are handy for having solutions to their problems. https://www.amazon.com/Schaums-Outline-Digital-Processing-Outlines/dp/0071635092?ie=UTF8&*Version*=1&*entries*=0