Rather than a primarily Tau novel like the name infers this is actually a Tyranid adversary novel. Commissar Cain is once more thrown into the deep end with dubious allies and worse enemies.
Brief synopsis of the Tau role - The Tau are giving an Imperial world a good shoeing. The world is special as it's a warp nexus and thus its lose would greatly impact the sector's logistics abilities. The Tau send an invasion force which promptly gets stuck into the local garrison. Imperial help comes with a relief force led by Cain which greatly bolsters the defenders. All looks fine until the naval force is called off elsewhere and, as always, just after they leave is when the Tau reinforcements also arrive. The Imperial forces get greatly pushed back and it's not long till the central command center is being targeted. Cain, knowing it's all lost except for the dying tries to convince the Guard commander that retreat is a better option than extermination and it's soon after that the Tau, as Cain predicted. decide to cut the Guard command structure out of the loop with a direct attack. Cain states this is Tau doctrine to envelope fortified areas and reduce them from range while denying relief and retreat. It's during the Tau attack that Cain is confronted by numerous Tau forces (primarily battlesuit) and is cornered by a Crisis suit that instead of attacking talks to him directly about a ceasefire. Knowing he was on the battlefield, the battlesuits had been using visual recognition software to locate him.
Even at the point of victory the Tau want to call it all off. The reason - Tyranids.
A Tau naval scout force ran into a portion of a hive fleet that shouldn't have been there. Unable to escape, the flagship of the scout force releases a messenger drone and detonates (they think) its own engines. Unfortunately, as other scouts report, the hive fleet has adjusted its course to follow the messenger drone and retreating scouts. Hence the cease fire as the Tau know their own and the Imperial forces are going to be needed for the coming battle(s). A reverse Damocles, as it were.
As noted by Cain and others, the Tau are not known for treachery when it comes to ceasefires and surrender agreements. What they are known for is following the letter of an agreement to their own advantage no matter what the spirit may have been. Also of great annoyance to the Imperial forces, when Tau refer to merchant vessels they actually mean heavily armed warships that also happen to coincidently have a fairly decent cargo capacity. This is considered cheating by Imperial diplomats but usually fully within the letter of the law as mentioned previously.
Human auxiliaries are observed are directly encountered by Cain. Even the existence of these people, who have grown up on tau conquered worlds, almost causes Cain to be ill and is seen as a great abomination. Known as "Facilitators," these human aux help a captured world adjust to their new status in the empire from a friendly face. Wisely, the Tau keep them mostly hidden from the Guard and Navy as riots would soon result as mentioned by Cain and others between their own horrified responses.
Negotiations are held on a Tau captured orbital causing the Imperials to be disgusted by the clean lines and curved surfaces shown within the newly Tau retrofitted areas. Aesthetic differences seem to be a common motif in both this book and the Shadowsun version.
Negotiations lead Cain and crew to reinforce a forgeworld while the Tau defend their own territories. Which begins the expected conflict between the Adept Mech and reinforcement forces.
Rail weapons are shown with slightly more realism. Rail rifle shots blow straight through a light armour vehicle and railgun shots are shown to cause disruption even after missing due to having a hyper-sonic projectile blow closely passed you. I can imagine a great deal of hearing loss after a Tau battle.