The answer is: Luke

Acts 1:1 / Luke 1:3 - showing Acts to be a subsequent work of Luke.

The author of the book of Acts is indeed Luke, who is also credited with writing the Gospel of Luke. It’s fascinating to notice how these two works connect, as both share a style and a purpose. Luke is sometimes attested as a physician, interestingly described in Colossians 4:14, which adds an intriguing layer to both texts—a dedication to careful observation, much like a doctor’s attention to detail.

Now, when we consider the overall message of Acts, it's all about the burgeoning early Christian community and its active journey, which kicked off following Jesus's resurrection and ascended into Heaven. As the sequel to the Gospel, it picks up the narrative with reflections on the spread of teachings and is rich with stories of remarkable figures like Peter and Paul, who helped establish the faith across diverse regions.

One fun piece of trivia is that Acts is often referred to as the "Gospel of the Holy Spirit" because it emphasizes the empowering role of the Spirit in the lives of believers. You can find echoes of prophecy from the Hebrew Scriptures throughout, invoking themes of restoration, inclusivity, and the rollout of divine purpose which speaks to a larger audience beyond traditional boundaries. Famously, in Acts 2 on Pentecost, we see how the Spirit beautifully unifies and empowers the early disciples to share the message in multiple languages—a precursor to the global splendor that faith has today!