The Spinward Marches and The Solomani Rim). A "book" was an expansion of some aspect of the original three little black books that comprised the game. In a sense, they were an early example of the "core rules" concept lately championed by several contemporary RPGs -- later additions to the game elevated to the status of being equal in importance to its initial rulebooks.
I didn't discover Traveller until 1980 or '81 by which point there were already several examples of all the categories of support material. I had only a spotty collection of both adventures and supplements, but I owned every single book produced for it. That's because, unlike adventures and supplements, I viewed books as Official™ and thus absolutely essential for running and playing a "proper" Traveller campaign. In retrospect, it's an absolutely foolish notion, but, having read one too many editorials by Gary Gygax on the importance of buying and using only official AD&D products, the mentality was very easy to adopt with regards to other games.
Fortunately, when I started playing Traveller, there were only two additional books beyond the three that started the game. The first of these was Mercenary, published in 1978 and written by Frank Chadwick and Marc Miller. Mercenary was 52 pages long and provided lots of new rules and information for ground forces, both Army and Marines. So you got lots of news military equipment and vehicles, a discussion of mercenary work, and a bunch of sample adventure outlines for such characters. But the big draw of Mercenary (and its immediate successor, High Guard) was its expanded character generation rules.
For those of you who don't know Traveller well, here's a brief overview of character generation. Characters can enlist in one of six services, three of which are explicitly military (Army, Navy, Marines). They then go through 4-years terms of service during which rolls are made to see what skills, if any, they acquire during that term. The most skills a character can acquire in a single term is four (in his first term if the player rolls very well), but most terms grant only one or two skills. Once the character musters out, he'll have a handful of skills that reflect his time in his chosen service.
Mercenary changes this somewhat by providing lots more detail about a character's terms of service. Each term represents a four-year block, but Mercenary lets the player roll for each year within that term, to see not only what a character's specific duties were during that time but also provides many more opportunities to gain skills. The result is that a character generated under the rules of Book 4 are almost always much more talented than those generated under the original three LBBs.
It's no surprise, then, that, by the time I started playing Traveller, nearly everyone I knew used Mercenary (and High Guard) to create their characters. This meant almost no one played Scouts or Merchants, at least until they, too, got expanded character options later in the '80s. After all, why wouldn't you use these rules? Not only did they generate better characters but they were also official. By calling Mercenary "Book 4," GDW had placed it on the same footing as Books 1-3 that came in the boxed ruleset. Unless you were explicitly trying to get a reputation for stubbornness, the pressure on the referee to allow Mercenary was significant.
At the time, I don't remember having any qualms about allowing Book 4 into our campaigns, despite the weird way it warped them into focusing almost exclusively on mercenary tickets and the like. Of course, this may explain why some of my players wanted to move on to play a "real science fiction game" like Star Frontiers, since its adventures weren't just about traveling from world to world fighting local insurgencies. Nowadays, though, I have a less positive feeling about Mercenary, feeling that it inaugurated not only power inflation in Traveller but a cult of the official that was common enough that a little pushback against it was much needed -- especially by guys like me whose natural tendency was to look to a game's designer/publisher for guidance rather than relying on my own judgment. Oh, the follies of youth!