Book 4: Mercenary set the stage for Traveller's continuing emphasis on the military. It was a natural expansion of two character types: Army and Marines, and led ultimately to the Traveller miniatures rules set: Striker.

For Classic Traveller, the term Books has a special meaning, presenting additional rules on specific subjects, expanding on Traveller's basic concepts. Books run 48 to 56 pages and may be used independently or together, but all require the basic rules sets.