Copy and Paste the notebook object twice. Set the copied shape's fill color to dark tan (#a8783f). Align one of the copies with the notebook shape created previously and place the other copied shape to the lower right of the rest of the icon.
Reduce the Opacity of the rightmost copied shape to 0% in the Transparency panel. Select both copied shapes and use the Blend Tool (W) to create a blend of 50 steps (or whatever amount gives your shapes a smooth blend).
Place the blend group beneath the notepad shape in the Layers panel.