Confirm your 16-inch wide space and gather all your materials. You'll be building a bookshelf that's 15.5 inches wide to allow a little wiggle room for fitting it into place.
Cut your pine boards to the following dimensions: two side panels at 36 inches tall by 8 inches deep, three shelf boards at 14 inches wide by 8 inches deep (these will sit between the sides), and one top piece at 15.5 inches wide by 8 inches deep.
Sand all your cut pieces starting with 120-grit sandpaper to remove rough spots, then finish with 220-grit for a smooth surface. Pay extra attention to the edges and any saw marks. Wipe away all dust with a tack cloth.
On the inside face of each side panel, measure and mark where your three shelves will go. Mark lines at 1 inch from the bottom (for the base shelf), 12 inches, and 24 inches. These will be your shelf placement guides.
Lay one side panel flat and position your bottom shelf against it at the 1-inch mark. Pre-drill two pilot holes through the side panel into the end of the shelf, then secure with wood screws. Repeat for all three shelves, then attach the second side panel the same way.
Place the top piece on top of your assembled frame so it sits flush with the back and overhangs equally on both sides. Pre-drill and secure with wood screws through the top into the side panels, two screws per side.
Fill all visible screw holes with wood filler and let it dry according to package directions. Once dry, sand the filled areas smooth with 220-grit sandpaper until flush with the surface.
Apply your chosen finish — wood stain followed by polyurethane for a classic look, or just clear polyurethane to show off the natural pine grain. Apply thin coats with a brush, letting each coat dry completely before adding another. Two to three coats will give you good protection.