Adam Clarke on Genesis 5:1

The book of the generations - ספר sepher, in Hebrew, which we generally translate book, signifies a register, an account, any kind of writing, even a letter, such as the bill of divorce. Here It means the account or register of the generations of Adam or his descendants to the five hundredth year of the life of Noah.

In the likeness of God made he him - This account is again introduced to keep man in remembrance of the heights of glory whence he bad fallen; and to prove to him that the miseries and death consequent on his present state were produced by his transgression, and did not flow from his original state. For, as he was created in the image of God, he was created free from natural and moral evil. As the deaths of the patriarchs are now to be mentioned, it was necessary to introduce them by this observation, in order to justify the ways of God to man.