(12) And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.
(15) if anyone’s name was not found written in the book of life, he was thrown into the lake of fire
To me, verse 15 if it's to be understood logically, then it leads to a sense of a possible condition, where : "maybe there is none - maybe there is at least one name is found written in the book of life".
But this possible condition is ruled out as in my own understanding, The Great White Throne Judgement is the second resurrection which happen to the non-elect, while the first resurrection is already happened before to the elect.
If I myself answer the question, then maybe the answer is something like this:
The Book of Life is there to show them that since the beginning of time, their names are not in it.
But since it's my own answer, I would like to know what is the Calvinist's answer.