Alex & The Immortals - The Immortal Realms, #1

Teleported to Asteroid Eros, Alex and his friends learn ancient arts like bottling lightning and mental telepathy from Pegasus, Cyclops, sea monsters, and gorgons. The Dark Master has sent an evil, shape-shifting amoeba to devour our planet. While Alex is cocky enough to lead his friends into battle, it is his brother, Dillon, who is marked for death. In the new worlds they travel to and the glorious sights they see, the children's friendships will be sorely tested.
They will make mortal enemies who would steal their souls if given half a chance. The bonds of brotherly love have never been more important than when the battle cry is sounded. The fate of the world, and his brother's life, rests upon Alex's scrawny shoulders.