Mary Pope Osborne - Buffalo Before Breakfast

Book 18 in the Magic Tree House series (1999)
Morgan Le Fey, a magical librarian from the time of King Arthur, has charged a brave young pair of children with the task of freeing an enchanted dog from a spell by collecting four gifts. In the 18th easy-to-read chapter book in Mary Pope Osborne's Magic Tree House series, eight-year-old Jack and seven-year-old Annie travel back almost 200 years to the Great Plains to find a gift from the prairie blue. Along the way, Annie and Jack make friends with young Black Hawk, narrowly miss a buffalo stampede, and learn about how the Lakotas view the earth and their place in it. (Ages 8 to 12)