list of books for digital communication
Digital Communications now cover a very large area of interest: Modulation, Channel Coding, Source Coding, Synchroniztion, Equalization, Diversity Techniques and a large number of specify applications (GPS, DVB-T/S, X-DSL, Optical Networks, ...). I think if You request opinion on "the best book" on this subject, You must prior study the basis who stands behind all the "scene".
I suggest to study WELL Haykin's "Signal and Systems", Papoulis' "Probability, Random Variables and Stochastic Process" and take a read to great Cover's "Elements of Information Theory".
Finally, I think it's impossible say "what's the best book on..." I think Haykin's "Communication Systems" is a great book but Proakis' "Digital Communications" is great too!
However I consider these two book how an Encyclopaedia of this subject and how all Encyclopaedias, the risk is to forget a deepenly (it's impossible by space and cost cover every topic with the same accuracy) tractation on specify arguments.