Endpaper illustration by Ben Kutcher from the 1925 edition of Oscar Wilde’s House of Pomegranates 1925
end·pa·per: a once-folded sheet of paper having one leaf pasted flat against the inside of the front or back cover of a book and the other pasted at the base to the first or last page.
For a more in-depth definition see the Etherington & Roberts entry in their bookbinding and conservation focused Dictionary of Descriptive Terminology.
Romeo Looks For a Job by Alain Gree 1966
There is an online show with 70+ examples of endpapers hosted by Nancy Stahl at Drawger.com, a website “where illustrators are writing about whatever it is they’re writing about at the time, and showing stuff as well.”
From Bob Staake’s soon to be released Mary Had a Little Lamb