A search on Biblegateway gives some 8 useages of the term 'Book of Life in the bible in the NIV, all but 2 being in Revelation. However the usage seems clear; it's a image of God having a clear knowledge of those who are His, who will be going to 'heaven'.

Paul's use in Phil 4:3 suggests that it is a synonym for being a Christian. The Revelation passages suggest likewise, though it is interesting to contrast:

And I saw the dead, great and small, standing before the throne, and books were opened. Another book was opened, which is the book of life. The dead were judged according to what they had done as recorded in the books.
Rev 20:12

with

Nothing impure will ever enter it, nor will anyone who does what is shameful or deceitful, but only those whose names are written in the Lamb’s book of life.
Rev 21:27

Although the first points to the reality that our status after death is not a result of our deeds, the second is a reminder that a persistence in doing evil proves that we are not on the way to heaven, and won't 'inherit the kingdom of God'. 1 Cor 6