Gregor Duncan illustrated nine books from 1939 through 1942, with five of them being done for Simon & Schuster. The books were in all genres from humor to adventure, and showed Duncan's breadth as an illustrator. The following are the books illustrated by Duncan, with examples in the gallery:

 

The Devil and the Doctor, by David H. Keller. Simon & Schuster, 1940.
Hail to Yesterday, by Mara Millar. Farrar and Rinehart, 1941.
The Melforts Go to Sea, by Geraldine Pederson-Krag. Simon & Schuster, 1941.
None But the Brave, by Rosamond Van Der Zee Marshall. Houghton-Mifflin, 1942.
Pardon Me for Pointing, by Arthur Kober. Simon & Schuster, 1939.
Run! Run! An Adventure in New York, by Harry Granick. Simon & Schuster, 1941.
Table for Four, by Jack Iams. Simon & Schuster, 1939.
Treasure Island, by Robert Louis Stevenson. Pocket Books, 1939.
Wacky the Small Boy, by Fred Schwed, Jr. Simon & Schuster, 1939.

 

Gregor Duncan's original illustration work is rare in general, but this is the first original book illustration that I've run across. The detailed pen & ink drawing is from Rosamond Van Der Zee Marshall's The Melforts Go to Sea, by Geraldine Pederson-Krag.  Published in 1941 by Holiday HouseThe Melforts Go to Sea, by Geraldine Pederson-Krag.  Published in 1941 by Holiday HouseOne of Duncan's illustrations from Robert Louis Stevenson's Treasure Island, 1939Wacky the Small Boy, by Fred Schwed, Jr.  Published by Simon & Schuster in 1939.  This was Duncan's favorite bookThe original cover illustration for Wacky the Small BoyAn original ink illustration from Wacky the Small Boy, featuring a sick WackyAn interior illustration from Wacky the Small Boy, showing Wacky fully in command