"This is the book of the generations of mankind on the day God created Adam, in the image of God He made them."
The word "Sefer" - which means "book" - is related to the word "sfar" which is the one of the Talmud's terms for "border" or "limit". Just as soldiers are generally stationed at a country's border, in order to guard it against foreign intruders, so too does a human being have a border that must be protected, and that is the "Image of God", which one must guard against foreign invaders, who wish to, God forbid, damage a person's "Image of God". This is the meaning of "This is the book", meaning this is the main border, "of the generations of mankind", that one must always be on guard that it should be always recognizable that "He made him in the image of God".
(Sefer Divrei Binah - Parshas Bereishis p. 24)