Christians please answer...?

If only you would keep reading. Go to Gen. 5 where it says that they had other sons and daughters.

There are no missing books of the Bible.

Cain's wife came from the Land of Nod, east of Eden.

Begin reading the Bible at Genesis chapter 1.

Gen. 3:20: “Adam called his wife’s name Eve, because she had to become the mother of everyone living.” (So all humans were to be the offspring of Adam and Eve.)

Gen. 5:3, 4: “Adam lived on for a hundred and thirty years. Then he became father to a son in his likeness, in his image, and called his name Seth. And the days of Adam after his fathering Seth came to be eight hundred years. Meanwhile he became father to sons and daughters.” (One of Adam’s sons was Cain, and one of Adam’s daughters must have become Cain’s wife. At that time in human history when humans still had outstanding physical health and vitality, as indicated by the length of their lives, the likelihood of passing on defects as a result of marrying a close relative was not great. After some 2,500 years of human history, however, when mankind’s physical condition had greatly deteriorated, Jehovah gave to Israel laws forbidding incest.)

Gen. 4:16, 17: “Cain went away from the face of Jehovah and took up residence in the land of Fugitiveness [or, Nod] to the east of Eden. Afterward Cain had intercourse with his wife [“knew his wife,” that is, intimately so, KJ, RS; “lay with his wife,” NE] and she became pregnant and gave birth to Enoch.” (Notice that Cain did not first meet his wife in the land to which he fled, as if she were from another family. Rather, it was there that he had sexual relations with her to produce a son.)

The books that are in the canon of scripture are the ones judged authentic. There are "other" books, not judged authentic, that are not there.

We don't know all the answers to all of our questions, even with the bible that we have. But as scripture tells us, it does contain those things necessary for salvation and godly living.

(Groans and Thunks forehead)

Where did you get the idea that all the people born in the first few hundreds or so years were written in the bible?

Adam and eve had losts of kids, those kids had losts of kids and those kids had lost of kids....and in a few years you have a lot of people

who said that they ONLY bore cain and abel?