Textures such as leather pose interesting challenges in pen and ink. I will often use a fine sable-hair brush, water, and India Ink to add a wash to my illustrations. I will dilute the ink down with the water and apply it with the brush. Or I will apply the ink to the paper with a pen (I use rapidograph draftsmen/engineering pens because of their control and precision) - and before the ink is completely dry I will use a damp brush to "smear" or "dilute" the ink down to get the desired effect. I used this technique on this illustration.