Required Text
Course readings were taken from the following required text:
Gallager, Robert G. Principles of Digital Communication. New York, NY: Cambridge University Press, 2008. ISBN: 9780521879071.
For an earlier version of this book, see lecture notes in the OpenCourseWare course 6.450 Principles of Digital Communication, Fall 2006.
Reference Texts
The required text covers all the material in the course, but the following references can provide enrichment and additional examples.
1. Proakis, John G. Digital Communications. 4th ed. New York, NY: McGraw-Hill, 2000. ISBN: 9780072321111.
This has a larger variety of systems than the class notes, but in less depth.
2. Proakis, John G., and Masoud Salehi. Communication Systems Engineering. 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2001. ISBN: 9780130617934.
This is an undergraduate version of the above text.
3. Wozencraft, John M., and Irwin Mark Jacobs. Principles of Communication Engineering. New York, NY: John Wiley and Sons, 1965. Reprint ed. Long Grove, IL: Waveland Press, 1990. ISBN: 9780881335545.
Classic text that first developed the signal space view of communication.
4. Wilson, Stephen G. Digital Modulation and Coding. Upper Saddle River, NJ: Prentice Hall, 1995. ISBN: 9780132100717.
Another text covering mostly the material of the last two thirds of the course.
5. Gallager, Robert G. Information Theory and Reliable Communication. New York, NY: John Wiley and Sons, 1968. ISBN: 9780471290483.
Treats most of the topics here in a more advanced information theoretic treatment.
6. Cover, Thomas M., and Joy A. Thomas. Elements of Information Theory. 2nd ed. New York, NY: Wiley Interscience, 2006. ISBN: 9780471241959.
An excellent text on information theory.
7. Tse, David, and Pramod Viswanath. Fundamentals of Wireless Communication. Cambridge, UK: Cambridge University Press, 2005. ISBN: 9780521845274.
Excellent coverage of many topics in the last third of the course.
8. Goldsmith, Andrea. Wireless Communications. Cambridge, UK: Cambridge University Press, 2005. ISBN: 9780521837163.
An up-to-date text on wireless channels.
9. Strang, Gilbert. Introduction to Linear Algebra. 3rd ed. Wellesley, MA: Wellesley-Cambridge Press, 2003. ISBN: 9780961408893.
Gives clear explanations of many standard linear algebra results.
10. Madhow, Upamanyu. Fundamentals of Digital Communication. New York, NY: Cambridge University Press, 2008. ISBN: 9780521874144.
11. Barry, John R., Edward A. Lee, and David G. Messerschmitt. Digital Communication. 3rd ed. New York, NY: Springer, 2003. ISBN: 9780792375487.
Papers
The following paper by Shannon laid the groundwork for developments in communication theory:
Shannon, Claude E. 
"A Mathematical Theory of Communication. (PDF)" The Bell System Technical Journal 27 (July/October 1948): 379-423 and 623-656.