Alan Lee was another major illustrator to work on the Lord of the Rings films as he too had been creating illustrations for the books many years prior.
Lee stated that he generally stayed on the ‘safer side of the anduin’  while Howe designed more of the darker elements.
I generally think that Lee’s work appears softer than Howe’s, which has a stark contrast between ‘good’ and ‘evil’. Lee’s work is more subtle in showing this, he also gives a lot of detail to the scenery of his images.
The texture of the landscapes in his illustrations look very lifelike and definitely give a distinct impression on what the place would be like. It helps to make the story seem more real and historical, like the director wanted for the film.