Trouble Under Oz

Trouble Under Oz is a 2006 novel by Sherwood Smith, illustrated by William Stout and published by HarperCollins. It is a sequel to Smith's 2005 novel The Emerald Wand of Oz which is a further continuation of the Oz series started by L. [Frank Baum] in 1900.
Dori and Em, Dorothy Gale's modern-day descendants from the previous book, return in this second installment. Dori travels to Oz in answer to a summons from Princess Ozma, while Em remains behind to deal with their parents' impending divorce. Ozma asks Dori to travel with Prince Inga to the Nome Kingdom; there they will aid Prince Rik to take control of the throne and avoid a war. The adventurers must defeat an Iron Giant and confront monsters on their way; they meet mermaids and invisible children.
Smith uses characters and locations from several Baum books, including Ozma of Oz, Dorothy and [the Wizard in Oz], The [Emerald City of Oz], Tik-Tok of Oz and Rinkitink in Oz.