In looking at a few books illustrated by W. W. Denslow, I noticed an interesting progression in endpaper design. The earliest example I have is from 1898, in A Cruise Beneath the Crescent, one of the Rand McNally titles with a Denslow designed cover. He also provided illustrations, including an endpaper drawing that runs along the left side of the front cover. This is a street scene in black & white.
The following year, 1899, Denslow illustrated Father Goose, His Book for L. Frank Baum. Here again he provided a simple endpaper design running along the left side of the cover. This time there is color added, in the tan background and the orange bill of the goose.
In 1900, The Wonderful Wizard of Oz was published. Here, Denslow extended his endpaper design to cover the entire left side, as opposed to a smaller strip design. But by confining the Cowardly Lion between two trees, he almost gives the same impression of a narrow illustrated strip on the left side. Once again there is a tan background to the image, although no additional color is added.
In 1901, the final Baum/Denslow title was published, Dot & Tot in Merryland. Here, Denslow has extended to fill both endpapers with his design. Three colors are used - black, with a rich brown and orange.
Finally, in 1903 Denslow provided a new endpaper design for The New Wizard of Oz, this time another double page spread in two colors, orange and green. This is a break from the tan and brown tones of his earlier designs, and feels far less regimented in the way the flowers spread across the pages. By this time, he had also started publishing some of his own picture books with more colorful endpaper illustrations.
The first endpaper design is quite detailed and a basically straightforward depiction of an eastern street scene. From there the designs become more playful, appropriately for children's books, with the final design for New Wizard being the most appealing to my mind.