When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tonguesas the Spirit enabled them. (Acts 2.1-4)
Worthwhile reads:
Craig Adams, Spirit Baptism: Wesleyanism and Pentecostalism
Daniel Kirk, The Spirit of Easter
Brian LePort, Pentecostal and not Pentecostal
Jack Levison, Pentecost for the Rest of Us
Bo Sanders, A Funny Thing Happened on the Way to Pentecost
the archives near Emmaus 