Hercules and Geryon
Hercules had to steal the cattle of Geryon. But Geryon had three heads and three bodies, as you can see in the picture. So he was hard to fight.
Twelve Labors of Hercules (Step into Reading, Step 3), by Marc Cerasini. Very easy, for beginning readers.
The Story of Hercules (Dover Children's Thrift Classics), by Robert Blaisdell (1997). Easy reading, very cheap.
Hercules, by Nancy Loewen (1999). More sophisticated, with a look at how the myth was passed on and what it meant to people, as well as the story itself.
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire. (Look under Heracles).
The Myths of Herakles in Ancient Greece, by Mark W. Padilla (1998). By a specialist; serious stuff.