According to a Gloss the book of life is the same as the predestination of the saints. They are the same reality but the ideas are different. It should be noted that in olden times it was a custom to write in a register the names of those appointed to some duty or dignity, as soldiers and senators, who were enrolled in the palace. Now all the predestined saints are chosen by God for something great, namely, eternal life; and this appointment is called predestination. The record of this appointment is called the book of life: and this record is in the divine memory, because inasmuch as He appoints, He predestines; inasmuch as He knows it unchangeably, it is called foreknowledge. Therefore, this foreknowledge about the predestined is called the book of life.
But is anyone ever erased from this book? I answer that some are enrolled absolutely, and others in a qualified sense. For some are absolutely predestined by God to obtain eternal life, and they are enrolled indelibly. Others are predestined to have eternal life not in itself, but in its cause, inasmuch as they are ordained to justice for the present; and such persons are said to be erased from the book of life when they fall away from justice in this life.
Thomas Aquinas, Commentary on Saint Paul’s First Letter to the Thessalonians and the Letter to the Philippians, (Albany, NY: Magi Books, 1966), 112-113.