Start at Taarengrav Wayshrine and head southwest. You have to pass through a camp full of worm cultists. In the middle of the camp there is a barrow, surrounded by torches. Directly south of the barrow there is a wooden platform, with wooden crates and The Legendary Scourge lorebook on them. The skyshard is behind those boxes.
Southwest from the Taarengrav Wayshrine, Southwest from the Taarengrav map marker, on a wooden walkway to the Southwest of the camp.