Signals and Systems: A Fresh Look, Chi-Tsong Chen, 2009

This is a self-published book, you can buy a paperback from Amazon, or you can find a freely distributable copy on the internet.  Buy a copy, if you are interested, it's worth it.  There are at least 30 widely used books out there on this topic.  This one focuses on what you actually need to know.  It is well written and classroom tested.

From the preface:

Feedback from graduates that what they learned in university is not used in industry prompted me to ponder what to teach in signals and systems. Typical courses on signals and systems are intended for sophomores and juniors, and aim to provide the necessary background in signals and systems for follow-up courses in control, communication, microelectronic circuits, filter design, and digital signal procession. A survey of texts reveals that the important topics needed in these follow-up courses are as follows:

• Signals: Frequency spectra, which are used for discussing bandwidth, selecting sampling and carrier frequencies, and specifying systems, especially, filters to be designed.
  
• Systems: Rational transfer functions, which are used in circuit analysis and in design of filters and control systems. In filter design, we find transfer functions whose frequency responses meet specifications based on the frequency spectra of signals to be processed.
  
• Implementation: State-space (ss) equations, which are most convenient for computer computation, real-time processing, and op-amp circuit or specialized hardware implementation.
  These topics will be the focus of this text. 
  

Dr. Chen's other books are also excellent.