Genesis 4:17, Who was Cain's wife?

And Cain knew his wife; and she conceived, and bare Enoch.  

//But where did Cain find her? It's a tricky question, but it does have an answer! Let me guide you there in a roundabout way.

We have at least two legends about man's arrival in the Bible, by two different authors. These two stories have been spliced together in Genesis. For more about how we know they were originally two separate stories, I refer you to the Documentary Hypothesis.

In one story, God creates the universe in six days. On the final day, God makes mankind, men and women both, and sends them forth to multiply. Kind of a boring story.

The second story is a bit more interesting. It begins in Genesis 2:7. God doesn't make mankind, he forms a man, (Adam) sculpting him out of the dust of the ground. God has planted a garden, and wants somebody to tend the garden. 

Story 1: All future generations descend from Adam. Story 2: No such assumption is made; Adam isn’t necessarily numero uno.

Suppose we keep reading in the Bible, past these two legends. One day, Adam's son Cain shows up with a wife! If Adam was the first-ever man on earth, then where on earth did Cain find her?

Story 1: Presumably, Cain hangs around and chooses a sister. Story 2: Probably, Cain chooses a wife from the heathen nations rather than choosing one of Adam's daughters. So which one is it?

Story 1: in Genesis 5:3-4, When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth. After Seth was born, Adam lived 800 years and had other sons and daughters.* There isn't any first-born Cain in this story. Repeat: No Cain in story 1. The firstborn is Seth. Story 2: Adam eats of the tree of knowledge, which apparently results in a sexual awakening, and lo! Children. Can you imagine their surprise? Anyway, as Genesis chapters 3 and 4 make clear, the first startling arrival is a fellow they name Cain. The second-born, Abel, comes along quite routinely. As the story continues, one child (Cain) kills the other and is driven away from the family. Cain pleads for mercy: "Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me." Cain seems to know that there are plenty of other people waiting on the other side of the mountain.

So, there you have it. Cain belongs to story 2, and probably found his wife among the nations of the world.

(*Note: Likely, the story of Seth as the firstborn of Adam is of yet another source, possibly an earlier source than Genesis chapter one, but may have been known to the writer who penned his six-day creation story.)