The Sea Fairies by L. Frank Baum

According to Wikipedia: "Baum had decided to end the Oz series with The Emerald City of Oz in 1910, after six installments over the first decade of the twentieth century. The Sea Fairies was intended to be the first in a new series of fantasy novels, which Baum and Reilly & Britton continued the next year with Sky Island. Unfortunately for author and publisher, the two volumes of the new projected series did not meet with the same success as the Oz books previously had. The first edition of The Sea Fairies sold 12,400 copies in its initial year on the market, where The Emerald City of Oz had sold 20,000. Even when Baum's books experienced a major resurgence in interest and sales in 1918, The Sea Fairies sold only 611 copies that year while the Oz books and even Baum's non-Oz works were selling thousands of copies. Once Baum returned to writing Oz books with The Patchwork Girl of Oz in 1913, the Trot series was retired — but the main characters lived on. Trot and Cap'n Bill are the main protagonists in The Scarecrow of Oz (1915) — the plot of which was reworked from the projected third book in their aborted series — and they play a significant role in The Magic of Oz (1919). Trot appears in The Lost Princess of Oz (1917) and Glinda of Oz (1920) as well."