One of the best books i've ever read is Lewis Carrol's Alice in Wonderland/Through the looking glass. When i read it first as a child, i would have been completely confused if it wasnt for John Tenniel's illustrations.
I had seen the Disney film before reading the book, and so was used to seeing the bright, colourful animation with the sweet and innocent main character, Alice.
John Tenniel's illustrations were a stark contrast, in black and white with surreal looking characters and dark scenery. Lewis Carrols Alice was an odd character with seemingly schizophrenic tendencies and Tenniels illustrations seemed to perfectly capture the bizarre world around the slightly unhinged little girl.
As the story is so fast paced and changes the scene and characters around Alice so often, it was only by looking at the incredibly detailed drawings that i was able to follow it.