- Simon Haykin, Digital Communications. The most clearly written book in the field. If you read just one, read this.
- Proakis & Salehi, Digital Communications. Comprehensive reference, every formula you will ever need.
- Sklar, Digital Communications: Fundamentals and Applications. Excellent intuition, especially for engineering tradeoffs.
- B.P. Lathi, Modern Digital and Analog Communication Systems. Gentle, readable; good if Haykin and Proakis feel dense.
- Cover and Thomas, Elements of Information Theory. For deep information-theoretic foundation.
- Lin and Costello, Error Control Coding. The classic algebraic-coding book. Modern editions cover LDPC and turbo as well.
- Richardson and Urbanke, Modern Coding Theory. LDPC and iterative decoding from the ground up.
- Andrew Viterbi, CDMA: Principles of Spread Spectrum Communication. From the inventor of the Viterbi algorithm and Qualcomm co-founder.
- Ziemer and Peterson, Digital Communications and Spread Spectrum Systems. Pragmatic, with many worked examples.
- Rice, Digital Communications: A Discrete-Time Approach. Modern DSP-flavoured treatment, very synced with how real receivers are implemented.
Once these click, Chapter 17 (DSP) and Chapter 23 (advanced communications) will feel like extensions instead of new material. Chapter 24 on hardware security will then turn the same techniques inside out, using the receiver math to attack rather than to communicate. Each chapter is the same math viewed from a different side.