What to read after History of Scotland?

The Romans, who had already made themselves masters of South Britain, were led into the northern part of the island by Julius Agricola, a.d. 80. But the Celts whom they found there, and whom they calledCaledonians, were so well able to defend themselves among their mountains that the Romans, though they defeated them in a great battle on the Highland border, gave up the idea of conquering the country, and retreated again south of the Firths of Forth and Clyde. Across the isthmus between the two, which is about thirty miles wide, they built a line of forts, joined by a rampart of earth. This rampart was intended to serve as a defence to their colonists, and as a boundary to mark the limit of their empire; though, as many Roman remains have been found north of the isthmus, they must have had settlements without as well as within the fortifications...