The Little House on the Prairie books were originally published starting in the 1930s with illustrations done by Helen Sewell. The original illustrations were done more in a fairy tale style rather than something that evoked more real life and personal connection by the reader. You can view the original illustrations here.
Garth Williams was commissioned to re-illustrate the entire Little House book series in order to make the illustrations seem more realistic rather than the fairy tale style the previous illustrator had done.
Williams actually went and met with Laura Ingalls Wilder to discuss the books with her. He went and travelled around to the different locations where the actual Little House books took place. This brought much realism to the new illustrations he had done and he didn’t shy away from portraying some of the harsher events that happened in the books.
Since the books were published with his new original illustrations, they have been re-released in a few different formats, including a full-color version of each book that is still available today for those who want to see illustrations in a bit of a different light from the traditional black and white that we all are familiar with.