Historical or, more properly, pre-historical fiction -- the setting is the British Isles way, way, way back when; the native inhabitants are dark-haired and use stone tools (shades of Robert E. Howard's Picts, although given a much more sympathetic treatment), but then from across the sea come blond-haired invaders with their copper swords ...
Less action than one might expect, given the description (although there are some moments of it), but with an excellent sense of place & time and a portrait of what these people's lives were like back in the day.
(n.b. This was written in the 1950s and I have no idea how well the scholarship holds up; but it's still a good read)