Viking Saga Series by Henry Treece

Harold is an adolescent who sails with his father on a raid to Scotland. They come to grief on an island in the Hebrides.

Harald is now a young man, his father has died. He has a complicated series of adventures which takes him to a giant's treasure cave in Ireland, then Jebel Tarik (Gibraltar), then to Miklagard (the Viking's name for Constantinople) where he joins the famous Varangian Guard. Eventually Harald returns home to Norway via the great ship-portage on the Dnieper.