"The Spirit Within and the Spirit Upon" by Kenneth Hagin is a foundational book that explores the two distinct experiences of the Holy Spirit in a believer's life. Hagin clearly explains the difference between the indwelling Spirit (within) and the outpouring of the Spirit (upon), providing insight into the biblical basis and practical application of these experiences. This classic work is a must-read for those seeking a deeper understanding of the Holy Spirit's role in their lives.