1. Acts 1:1 (ESV)
  2. Exposition

To what book does the "first book" refer?

Acts 1:1 (ESV)

1 In the first book, O Theophilus, I have dealt with all that Jesus began to do and teach,

The first book refers to the Gospel of Luke. The writer, Luke, assumes that those reading the book of Acts will be familiar with the basic details of Jesus’ life as recorded in his (or another) Gospel. With this phrase, he links Acts with the Gospel of Luke.