According to the article "Sex Before Marriage: How Far is Too Far," by Tim Lane, which book of the Bible is all about sex?
The book of the Bible that is "all about sex" is the Song of Solomon. This book is also called the Song of Songs.
There is some degree of disagreement among Bible scholars as to whether this book is truly all about sex. It certainly does seem to be that way on first reading. However, there are those who argue that it really is meant metaphorically. These scholars argue that the book is about the relationship between Jesus and his people. It is put in romantic and sexual terms as a way of helping people to understand it.
However, in the context of the article that you are asking about, the clear answer is that Song of Solomon is the book about sex. None of the other books of the Bible is even remotely "all about sex." Lane discusses Song of Solomon as if he believes that it is literally about sexual love between married people.