Good Starting Material for a SEMI Beginner
Signal Processing First _ McClellan


Very readable introduction to signal processing.


Signals and Systems_Haykin


Probably the most readable undergraduate book on signals and systems.


Signals and Systems_Oppenheim


Classical text on upper undergraduate Electrical Engineering introduction to signals and systems. Not an easy read but definetely worth it. However, it's very critical to find a solutions manual for benefiting from this book.


Discrete-Time Signal Processing_Oppenheim


Formal introduction to the subject matter. Find a solutions manual.