You want to Begin and End each column with a Black 2 1/2" x 5" block. Also Pay Attention to which way the stripe is going when you cut and sew. "Books" tend to be on shelves vertical!
My quilt was long so I needed to sew 2 black strips together to make them long enough for the sashing. I sewed these strips straight together, not on the 45 angle. I did it this way because all the "angles" in the blocks and Sashing were Vertical - If you sew your sashing on the 45, those seams would look "odd" with the Verticalness (a word?) but how the Books and Shelves are vertical.