Luke wrote this part (as well as one Gospel) and it is the only written record describing the early Christian Church.
This history does appear however to be idealised. It suggests that the Christian Church may have developed in a more organised way than it actually did. Most of this book describes the journeys of the Apostle Paul.
This book is a series of forecasts. It describes the end of the world. It was written as encouragement to Christians who were being persecuted at that time. The Apostle John (who wrote one of the Gospels) was thought to have written this book, but this is now doubted.