Magic Tree House(#49): Merlin Missions #21: Stallion by Starlight

The #1 bestselling chapter book series of all time celebrates 25 years with new covers and a new, easy-to-use numbering system!
Hold your horses! Jack and Annie must find four secrets of greatness for Merlin the magician. To start, they travel back in time to meet Alexander the Great. He should know all about greatness, right? But young Alexander is bossy, vain, and not great at all! How can they learn from him? It's going to take a wild black stallion, magic from Merlin, and a lot of bravery to succeed. Are Jack and Annie clever and bold enough to complete their mission?
Formerly numbered as Magic Tree House #49, the title of this book is now Magic Tree House Merlin Mission #21: Stallion by Starlight.