Principles of Cognitive Radio, E. Biglieri, A. J. Goldsmith, L. J. Greenstein, N. Mandayam, and H. V. Poor
It is a very recent book (published in December 2012), which thoroughly studies the principles and fundamentals of Cognitive Radio. I believe it can be the first book to be taught at an undergraduate level.
Principles of Digital Communication, R. G. Gallager
This book is written by the renowned communications theorist R. Gallager. He has an incredible teaching style with a unique philosophical approach. A course on the principles of digital communications is taught by him at MIT, and it is available here. His video lectures can be found here
Principles Digital Communications over Fading Channels, M. K. Simon and M. Sl. Alouini
This book provides a unified framework for the performance analysis of digital communication systems under a variety of fading channels.Wireless Communications, A. J. Goldsmith
Wireless Communications: Principles and Practice, T. RappaportEven though it is a relatively old book, it explains the principles of wireless communications in a very neat way. It also provides a great analysis of different communication channel models.
Fundamentals of Wireless Communication, D. Tse and P. Viswanath
Fundamentals of Statistical Signal Processing (Volume I: Estimation Theory. Volume II: Detection Theory), S. Kay
Principles of Signal Detection and Parameter Estimation, B. LevyA new book that covers the fundamentals of signal detection and parameter estimation.
An Introduction to Signal Detection and Estimation, V. Poor
Convex Optimization, S. Boyd and L. Vandenberghe
Probability and Random Processes, S. Miller and D. Childersook
This textbook is really good for fresh graduate students who want to improve their mathematical background on probability distributions and the essential tools to analyze random and stochastic processes.