One of the main works on Pentecostalism is also one of the easiest reads: Vinson Synan's The Holiness-Pentecostal Tradition: Charismatic Movements in the Twentieth Century. This revision of the 1971 The Holiness-Pentecostal Movement in the United States will likely be as influential as the original.

Any student of Pentecostalism should own a copy of The Dictionary of Pentecostal and Charismatic Movements. It contains more than 800 entries (and 300 photographs) written and edited by the top scholars of Pentecostalism; it includes everything from biographical sketches to lengthy discussions of theological distinctives.

An overview of the Pentecostal movement, including a brief history and summary of beliefs, is available at the New Religious Movements area of the University of Virginia's Sociology department site.

For a more academic look, check out the Cyberjournal for Pentecostal-Charismatic Research. It covers Pentecostal history as well as current trends.