From the back cover:
The first rules supplement for the latest eidtion of the acclaimed Traveller roleplaying game. Mercenary is design to add to the existing system without damaging game balance and this book is a must for those who wish to seek adventure in the realms of private security.
Mercenary inclues new character careers in the shady world of the hired gun and "security consultant". From the outlaw guerilla to the elite commando and amoral Warmonger these new classes bring many new and varied options for Player Characters.
Rules for running a mercenary unit with a complete system on creating mercenary tickets is included as is an abstract mass-combat rules system. So characters have the opportunity to continue their mercenary careers with their own unit.
With an arsenal of new equipment now at the disposal of these dogs of war the universe just got a lot more dangerous!