In 1910, L. Frank Baum published The Emerald City of Oz. This was intended to be the last Oz book, and Reilly & Britton pulled out all the stops. The spine was stamped in silver, the cover label had both silver and green metallic ink, and there were 16 color plates printed from watercolors by John R. Neill. These plates also had highlights of metallic green ink, making this a very opulent book.
This label was soon replaced by a much simpler one, based on the endpapers of Dorothy and the Wizard in Oz. I never quite understood this - even if the metallic accents were dropped, the same image could have been used. I suspect that Reilly & Britton wanted to shift to a simpler, more graphic cover in keeping with the books being published at that time. Below I show the original cover in both light and dark blue cloth, as well as the secondary cover.
When I was a kid, I ran across a copy of Emerald City at a friend's house. At that point, I had only read the first two books of the series, and was thrown by the Nome King, a character I had not read of before. I don't think I even finished the book - the characters and story had evolved too far from what I already knew. Once I started filling in the gaps from the other books of the series, that wasn't a problem.
As a collector, The Emerald City was a book I was very anxious to get in first edition. It has all the bells and whistles of an Oz book, and aside from being a fun story, is a great object. At least some of the original watercolor illustrations survive today - I'm not sure how many. Like the Road to Oz illustrations, I consider these to be Holy Grail Oz illustration pieces.