Mal 3:16 Then they that feared the LORD spake often one to another: and the LORD hearkened, and heard [it], and a book of remembrance was written before him for them that feared the LORD, and that thought upon his name.
Mal 3:17 And they shall be mine, saith the LORD of hosts, in that day when I make up my jewels; and I will spare them, as a man spareth his own son that serveth him.
Mal 3:18 Then shall ye return, and discern between the righteous and the wicked, between him that serveth God and him that serveth him not.

Watch this very carefully. They spoke (fellowshipped) with one another often, they feared the Lord and constantly thought upon His name. Now there are all the saved, but there are these who go that extra mile for the Lord and are considered for the book of remembrance. What is this book of remembrance?

Those in the book of remembrance will be spared something when God begins to act at the consummation of this age of the Tribulation. Would not this be the first rapture (Rev. 7.9) in which they are spared the Tribulation? They would be accounted worthy to escape the Hour of trial that is to come upon the whole world because they were watchful, prayerful and kept the Word of His patience (Rev. 3.10, Luke 21.36).

This book of remembrance is not the book of life. They can not be saved by works, but they are rewarded by works. Then they are the ones who shall return (Jude 14,15) with the Lord to judge. Some believers will receive this reward to reign during the millennial kingdom, some will not, though still saved for the new city in the new earth after the millennium-to-come.

In simplest terminology we may say the book of remembrance is first rapture according to readiness.