John R. Neill didn't illustrate many books after the 19-teens, other than the Oz series. A notable exception to this is Peter and The Princess, which was published by Reilly & Lee in 1920. This title was was an elaborate gift book, and a showcase for John R. Neill's artwork, containing 8 beautiful color plates, plus color title page, endpapers, and cover - eleven images in all.
These illustrations were taken from finished watercolors, rather than ink drawings with color added by the printers, and the book was sold in a decorative box. I do find it a bit odd that Neill didn't create any ink drawings for the book, since he was such a master of that medium. But the watercolors are lovely, and I believe most of them remain in the Neill family today.
Two years ago I had an opportunity to visit with Jory Mason, one of Neill's granddaughters. This was a memorable occasion, as I was able to view a great deal of original artwork at the time. As a bonus, she had a large quantity of pieces belonging to her Aunt which were being returned shortly after my visit. Among these were several of the watercolors for this book, and it was a thrill to see them in person.