This book contains the third and fourth novels in the Russian Oz-inspired "Magic Land" series. In "The Seven Underground Kings," Ellie and her cousin Fred arrive by accident in the kingdom of the Underground Ore-Diggers at a time of crisis, and help put things right. In "The Fiery God of the Marrans," villainous Urfin Jus, using an ordinary cigarette lighter, passes himself off as a god to the primitive Marran people, who do not even have fire, and makes them into a powerful military force to use to conquer the Emerald City.
Librarian Note: There is more than one author in the Goodreads database with this name.
Alexander Volkov (Александр Волков) wrote several historical novels, but is mostly remembered for a series of children's books based on L. Frank Baum's The Wonderful Wizard of Oz. The first of these books, The Wizard of the Emerald City, is a loose translation of the first Oz book, with chapters added, altered, or omitted, some names changed (for example, Dorothy becomes "Ellie" and Oz is renamed "Magic Land"), and several characters given personal names instead of generic ones.
First published in 1939 in the Soviet Union, the book became quite popular; and in the 1960s Volkov also wrote his own sequels to the story. From 1963 to 1970, four more books in the series were published, with the sixth and final story published posthumously in 1982.