I am interested in learning more about the Pentecostal perspective. Which author(s) would you suggest starting out with?
For Pentecostal theology, I would suggest these works:
Foundations of Pentecostal Theology by Guy P. Duffield and Nathaniel M. Van Cleave (both pastors in the Church of the Foursquare Gospel). This would be good as an overview of the theology of Foursquare and Assemblies of God type groups.
For an overview of classical Pentecostal Pneumatology (the doctrine of the Holy Spirit), I would suggest Anthony Palma's
The Holy Spirit: A Pentecostal Perspective. Palma is affiliated with the Assemblies of God.
The first two books are more theological in tone. If you are looking for material that is more pastoral or devotional in nature, I would recommend anything by Jack Hayford. Specifically, take a look at his book
Living the Spirit Formed Life: Growing in the 10 Principles of Spirit-Filled Discipleship.
The above authors all come from the non-Wesleyan school of Pentecostalism. However, there is a lot of great work by Wesleyan Pentecostals as well. A classic written by Stephen Land of the Church of God (Cleveland, TN) is
Pentecostal Spirituality: A Passion for the Kingdom.
If you are looking for a pretty solid but brief academic overview of Pentecostalism, I'd recommend
Pentecostalism: A Very Short Introduction by William K. Kay and published by Oxford University. Kay is Professor of Theology at Glyndwr University and founding Director of the Centre for Pentecostal and Charismatic Studies.