Who can believe that this is number 39, yes, 39, in this popular series for young readers? Even readers who are unfamiliar with the series or the previous titles will be able to pick this one up without missing a beat. Siblings Jack and Annie are drawn to the treehouse by a golden eagle and then magically transported to Mongolia in 1992, where locals are eagerly awaiting the arrival of Takhi, wild Mongolian horses who have disappeared from their natural habitat and only found in zoos. Some of them are being returned to the reserve, Hustai National Park, to see if they can flourish and restock the area. The youngsters meet Tuya, a local Mongolian girl, who provides background on the wild horses. And then, at night, when part of the herd is threatened by wolves, Jack and Annie rescue them, with help from a mysterious, white stallion. As usual, the plot is simple, fast-paced, and ideal for its intended audience, and the artwork complements the text effectively. It just seemed less engaging and believable to me than some previous titles so it's a 2.5 for me. No matter, young readers will still love the book and series.