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I suggest that you start with this online textbook "The Scientist and Engineer's Guide to Digital Signal Processing" by Steven W. Smith, Ph.D. You can browse and/or download the entire book in PDF format here h**p://www.dspguide.com/pdfbook.htm
In particular, take a look at the section on DSP applications, namely Chapters 22-29. This should allow you to pick an application area of interest to you.