This primary textbook is recommended:
S. Haykin & M. Moher, Communication Systems, 5th Ed., John Wiley & Sons, 2009.
These additional textbooks are also acceptable:
B. Sklar, Digital Communications, 2nd Ed., Prentice Hall, 2001.
L. Couch, Digital and Analog Communication Systems, 7th Ed., Pearson Prentice Hall, 2007.
H. Stern & S. Mahmoud, Communication Systems: Analysis and Design, Pearson Prentice Hall, 2004.
J. Proakis & M. Salehi, Fundamentals of Communication Systems, Pearson Prentice Hall, 2005.