Space Viking should be required reading for Traveller players. You could say the same for just about any Piper book. The interesting twist in this one (don't worry, this doesn't rise to the level of a spoiler) is that, although the raiders act like Vikings, swooping in and killing and taking what they want, they become the civilizing force taming a barbarous galaxy instead of t'other way round.
The Dumarest novels, too, are essential if you play a non-Imperium Traveller universe. They're formulaic, so don't read them one after another, but also loads of fun, and the influence they had on Traveller leaps off the pages. They contain very little of what you would call "space action," however. The stories are almost completely planet-based, as Dumarest travels from one unique, exotic world to another in search of "Earth" and tries to stay one step ahead of the deadly Cyclan.
Steve