Part one is built around the Drake equation, which tries to separate out some of the probabilities involved in the overall probability of contact. McConnell doesn't try to calculate an actual result — the numbers are just too uncertain — but he provides a guide to the factors involved and some of the things that might influence them. He also touches on some of the possible explanations for the Fermi Paradox (why haven't colonising aliens or von Neumann probes populated the galaxy?).
Part two turns to the mechanics of communication. This covers radio and optical (laser) hardware and astrophysics, including a basic introduction to concepts such as spectra and bandwidth and noise and doppler shifts. It also covers signal processing and confirmation, with a chapter on the SETI@Home project.
Part three, taking up half of Beyond Contact, is a long and detailed account of one specific approach to communicating once contact is made. McConnell's basic idea is to communicate a simple programming language, so he starts with binary numbers, progresses through a kind of machine code and what he calls "igenes" (self-contained code fragments), and proceeds to simulations, pictures, concepts, semantic networks, and natural languages. And an appendix looks at the possibility (if there are enough communicative civilizations) of building a galactic "mesh", a store-and-forward network akin to Usenet, with a common protocol evolving by a kind of natural selection.
There are some interesting bits and pieces in Beyond Contact (for example encoding colour for a species which may have a radically different visual system), but a lot of it will be rather tedious for anyone with a computer science background. And while the physics is quite well explained, novices to programming concepts will find part three heavy going: there are definitely better general introductions to programming around. It doesn't help that McConnell persists in providing invented raw numbers all the way through part three, even when dealing with high level concepts — yes, this helps remind us that it all gets translated into numbers in transit, but that's hardly worth the extra complexity and verbosity. This part of Beyond Contact might have worked better as straight narrative text describing the general ideas, with the details and pseudo-code separated out so they could be more easily skipped (either as "too hard" or "too easy"). But check out the O'Reilly web site, which has two sample chapters as well as an article "Anticryptography" covering some of the material in Beyond Contact much more concisely.