The people you are talking about are called the Cantabri. They lived in what is now Northern Spain, which is a mountainous and hilly region. Notably, this was also the holdout area for the surviving Visigothic Christians after the muslim conquest of Spain in the (8th*) Century. ^^^^thanks ^^^^/u/DaftSaft That's an entirely different story, and their survival was for different reasons, but it goes to show that Northern Hispania was exceptionally difficult to take. One of the reasons for success seems to be the nearly unified resistance to Roman rule amongst the Cantabri, and allied Asturians, and them not committing to a massive battle against a combined Roman army. I will note that Florus writes of a combined confrontation briefly in his text, although the translation seems to be a little wonky, and Augustus himself was not involved.

As for there not being much surviving writing on the subject, there are many possibilities which are mostly speculation. The first is that there were several accounts, by eyewitnesses or later writers which haven't survived. Hispania, and especially northern Hispania, was quite outside the area of focus for Rome, and was less colonized than other regions in the empire, making for less physical information about the conflict (roman veteran gravestones make for great historical references). Another is that Augustus did not find his victory convincing enough to celebrate it, as the conflict was more focused on brutal pacification and guerrilla war, rather than a final decisive victory like Julius Caesar had at Alesia. Not that a decisive battle was needed for a triumph, but it was a political act that perhaps he didn't feel was worth it. At that point he was furthering his control over public displays of power, and triumphs were reserved for him and those he likely wanted to succeed him. The fact that he didn't celebrate a triumph suggests that this is a partial reason for the absence of information.