Objection 1. It seems that the book of life is not the same thing as pre-destination. For it is said, "All things are the book of life" (Sirach 4:32)--i.e. the Old and New Testament according to a gloss. This, however, is not predestination. Therefore the book of life is not predestination.
Objection 2. Further, Augustine says (De Civ. Dei xx, 14) that "the book of life is a certain divine energy, by which it happens that to each one his good or evil works are recalled to memory." But divine energy belongs seemingly, not to predestination, but rather to divine power. Therefore the book of life is not the same thing as predestination.
Objection 3. Further, reprobation is opposed to predestination. So, if the book of life were the same as predestination, there should also be a book of death, as there is a book of life.
On the contrary, It is said in a gloss upon Psalm 68:29, "Let them be blotted out of the book of the living. This book is the knowledge of God, by which He hath predestined to life those whom He foreknew."
I answer that, The book of life is in God taken in a metaphorical sense, according to a comparison with human affairs. For it is usual among men that they who are chosen for any office should be inscribed in a book; as, for instance, soldiers, or counsellors, who formerly were called "conscript" fathers. Now it is clear from the preceding (23, 4) that all the predestined are chosen by God to possess eternal life. This conscription, therefore, of the predestined is called the book of life. A thing is said metaphorically to be written upon the mind of anyone when it is firmly held in the memory, according to Proverbs 3:3: "Forget not My Law, and let thy heart keep My commandments," and further on, "Write them in the tables of thy heart." For things are written down in material books to help the memory. Whence, the knowledge of God, by which He firmly remembers that He has predestined some to eternal life, is called the book of life. For as the writing in a book is the sign of things to be done, so the knowledge of God is a sign in Him of those who are to be brought to eternal life, according to 2 Timothy 2:19: "The sure foundation of God standeth firm, having this seal; the Lord knoweth who are His."
Reply to Objection 1. The book of life may be understood in two senses. In one sense as the inscription of those who are chosen to life; thus we now speak of the book of life. In another sense the inscription of those things which lead us to life may be called the book of life; and this also is twofold, either as of things to be done; and thus the Old and New Testament are called a book of life; or of things already done, and thus that divine energy by which it happens that to each one his deeds will be recalled to memory, is spoken of as the book of life. Thus that also may be called the book of war, whether it contains the names inscribed of those chosen for military service; or treats of the art of warfare, or relates the deeds of soldiers. Hence the solution of the Second Objection.
Reply to Objection 3. It is the custom to inscribe, not those who are rejected, but those who are chosen. Whence there is no book of death corresponding to reprobation; as the book of life to predestination.
Reply to Objection 4. Predestination and the book of life are different aspects of the same thing. For this latter implies the knowledge of predestination; as also is made clear from the gloss quoted above.
Source: Summa Theologica (New Advent)