Line: The book shelf in the room shows the use of line because the book shelf shelves are built against the wall forming a miniature tree. Our eyes are drawn to look at the book shelf tree from bottom to top and outward to the tree/bookshelf branches.
This is a perfect combo of my two favorite ideas for that space under the stairs leading to the basement. A reading nook AND book storage and shelves . assuming that there is enough space for this there.