I mentioned that if I found any further information on the Nutshell Library (the one from the 1960's) I'd add a note to the blog...
Courtesy of Richard Mori of Mori's Books, there IS a fourth set available... by A.A. Milne -- it's four miniature books in the Winnie-the-Pooh series.
That brings the list to:
A. A. Milne
if you happen to know of more, please let me know.