Which book of the Bible discusses the concept of what happens to believers at death?

The book that discusses the concept of what happens to believers at death is Philippians. In Philippians, particularly in chapter 1, Paul expresses his thoughts on life and death, indicating that to live is Christ, and to die is gain. He conveys a sense of hope and assurance regarding the afterlife for believers, highlighting the expectation of being with Christ after death. This focus on the eternal perspective provides comfort and clarity about the fate of believers, which is a central theme in Paul’s writings.

While other books in the New Testament touch on similar themes, Philippians is distinct in its straightforward expression of a believer's relationship with life and death and the joy of being united with Christ beyond this life. This makes it the most relevant choice in regard to the question asked.