This year marks the 100th anniversary of The Sea Fairies, L. Frank Baum's attempt to break away from the Land of Oz and start a new series with the characters Trot and Cap'n Bill. The series only lasted through 2 books, with the main characters finally arriving in Oz in Baum's 1915 book, The Scarecrow of Oz.
Mermaids and life under the sea are the main components of this story, giving John R. Neill many opportunities for striking illustrations. He seems to have enjoyed the subject, as he provided another lovely image of a mermaid in his drawing of the whirlpool that captures Trot and Cap'n Bill's boat in The Scarecrow of Oz.
Off-hand, I don't know of any other Baum stories dealing with mermaids, although a mermaid with a parasol makes a brief appearance during an underwater sequence in the 1914 Oz Film Co. production of His Majesty, The Scarecrow of Oz.
This book was not a great success, but it did get reprinted over the years and went through several different cover designs. The original cover, which used a metallic gold background, was dropped quickly and replaced with an adaptation of one of the duotone color plates within the book. When Reilly & Lee reprinted this title in 1920, Neill created a new full color cover, shown on the left.
This was the same time period during which he illustrated Peter and the Princess (1920), and close in time to his illustrating Andersen's Fairy Tales (1923). These three titles seem to be the last book covers Neill produced as full watercolor paintings. He used very similar type styles for the titles of Sea Fairies and Andersen's.
In fact, the cover of Andersen's doesn't really seem to particularly apply to any of the stories within that book, but it does bear a striking resemblance to the final illustration from The Sea Fairies!