David Richardson has recently published the portraits of many of the characters appearing in ISOLT. The book is a limited edition (translation: very expensive) and the production quality is very high. Richardson is a vibrant colorist, recalling to my mind the portraits of Matisse. I have copied a few random portraits from each artist for you to make the comparison.
(The book, as well as individual prints, is available at http://www.davidwesleyrichardson.com/order/)