Acts is the only historical book in the New Testament that deals with the life of the church.
Normally a church history would focus on the internal life of the church — organization, theological disputes and development, church and culture. They aren't the main story. They stay in the background.
What is the main story? According to Harry Boer,
Acts is governed by one dominant, overriding and all-controlling motif — the expansion of the faith through missionary witness in the power of the Spirit.
Restlessly the Spirit drives the Church to witness, and continually churches rise out of the witness.
His book on Pentecost and Missions is a classic. If you're quick you can get a second-hand copy for $2.