Cybele, Magna Mater or the Great Mother, was a West Asian fertility goddess like Isis or Demeter. You sacrificed to Cybele to make sure the wheat and barley would grow well. There are many versions of this story, but according to Ovid‘s story in the Metamorphoses, Cybele fell in love with a beautiful young man, Attis, who lived in Phyrgia, in what is now south-west Turkey.
Cybele demanded that Attis be completely faithful to her, but a nymph (like a fairy) called Sagaritis kept trying and eventually convinced Attis to get with her. Cybele was super angry when she found out. She killed the nymph and made Attis go crazy and cut off part of his body. He died, and grew again as an evergreen pine tree.
Ancient Mesopotamians, by Elena Gambino (2000). For kids, retellings of Mesopotamian stories and lots of context.
Gods, Goddesses, and Monsters: An Encyclopedia of World Mythology, by Sheila Keenan (2000). Easy reading.
Gods, Demons and Symbols of Ancient Mesopotamia : An Illustrated Dictionary, by Jeremy Black and Anthony Green (1992).
Religion in Ancient Mesopotamia, by Jean Bottero (2001).