Fun With Washi!
Run a piece of washi tape across the entire front of your bookshelf. With scissors, trim the ends of the tape for a clean edge.
Place a piece of the complementary washi tape, about 12 in/30.5 cm long, on a slightly larger piece of parchment paper. Measure the width of your tape (mine is 5/8 in/16 mm), and mark the piece of tape in increments of that measurement along the full length. Cut the washi tape to make squares.
Peel of one square from the parchment paper and rotate it 45 degrees so that it looks like a diamond. Place it at this angle on the far left side of the washi-covered shelf front.
Peel off another square and place it beside the first diamond. Repeat, until the entire shelf front is covered with a harlequin pattern.