Acts of the Apostles
The book of Acts is the first book of the New Testament after the Gospels. It tells the story of the early church from the ascension of Jesus to the house arrest of Paul in Rome. The opening of Acts refers to "the first book" and, like the Gospel of Luke, is addressed to a man named Theophilus. This makes it highly likely that Acts was written by the same person as Luke.
Contradictions in Acts
In the account of Paul's conversion in chapter 9, it says that Paul's companions heard Jesus' voice but did not see anyone. However, in chapter 22, Paul says that his companions saw the light but did not hear the voice. It seems the author of Acts believed they had gotten half of the experience, but could not keep straight which half.