Sketch the cover
Draw a rectangle at a slight angle. For a closed book, add a second offset line along the lower and side edges to suggest thickness.
Shape the spine
Connect the corresponding corners with short lines. Curve the spine edge slightly if you want a soft cover; keep it straight for a more rigid hardback.
Add a few details
Use two or three light parallel lines for the page block. Add a title area and shade one side. Keep the detail simpler than the outline so the object remains readable at a small size.