The dead who stand before God could not include true Christians today, because they will appear before Christ and receive their rewards at the first resurrection when He returns. It could not refer to those converted during the Millennium, because they will have already inherited the Kingdom during the thousand years, after living a normal life span.
Those in this second resurrection are those who died in spiritual ignorance in past ages. Their time of salvation occurs after the second resurrection!
The Greek word translated "books" in verse 12 is biblia. It is from this word that the English word "bible" is derived. The books that are opened are the books of the Bible! They will be opened to the understanding of the multiple thousands of millions of people who had not been able to know and understand God's purpose and plan for them during their previous lifetime, never having been called by the Father and given His Spirit of understanding.
Some, assuming that "judgment" means only the pronouncing of a sentence based on past behavior, have thought that these people, even if they repented, could do nothing to alter their fate. Thankfully, God is merciful, always willing to forgive upon true repentance.
In this period of judgment, not only is the Bible opened to their understanding, but the Book of Life is also opened. These people are given an opportunity to receive eternal life! God, in perfect fairness, will give them the same opportunity for salvation He now gives to those He calls in this age. However, they will not be subject to Satan's influence as we are today, because Satan will have been finally sent to his punishment (Revelation 20:10).