It updates several of the campaign-style game rules. I'm not very familiar with all of the old City of Death, Planetstrike, or Cities of Death rules, so I can't comment about how they've changed, but I've got a copy of the book and am willing to answer Qs (within reason with regard to the Forum Rules) if you got them.
Most, if not all, of the fortifications are reprinted here, and though most are Imperial-flavored, every datasheet in the book is available to take in Tau Empire detachments. If you skipped Kauyon in favor of getting only the Tau Codex, you can add all of the Tidewall (which are still Tau exclusive) (and more) datasheets to your collection with this book.
Of note, regarding fortifications, you can buy extra woods or ruins for 20 points. I haven't ever played with fortifications before, but apparently you can also give many buildings a voidshield for 25 points, among other options.
All of the Tidewall parts and networks appear here, including the "exclusive" Tidewall Rampart. The Rampart's datasheet is a much, much easier read than the insert was that was included in the model kit. The set-up of the Rampart has changed, or at least been made much clearer. I more or less consider this errata, but I understand if a mod feels the need to remove this. The new set-up is as follows:
"When setting up a Tidewall Rampart, all of the Shieldlines must be set up in a continuous line, each touching at least one other Shieldline in the network. Each Gunrig must then be set up within 6" of one of the network’s Shieldlines, and each Droneport must be set up within 6" of one of the network’s Gunrigs."
It's a lot cleaner of a set-up, and a little less cheesable (not that the Rampart was being brought out for cheese play very often), but it's funny to note that the Datasheet's image shows an illegal Rampart arrangement.