(It also was relevant to the entire “does God exist?” debate – an eventually cruxy point for me is that you totally can build up simulations of people in your head, and I’d expect that to be hard to distinguish from God speaking to you)
Luhrmann specifically examines how evangelicals come to experience God as a close, intimate, and invisible but very real friend and confidant with whom they can communicate on a daily basis through prayer and visualization, clearly recognizing His voice. [...]
Luhrmann investigated the new evangelical movement as a participant-observer. She attended services and small group meetings for several years at local branches of the Vineyard, an evangelical church with hundreds of congregations throughout the country and the world, and had hundreds of conversations with evangelicals, learning how they believed themselves able to communicate with God, not just through one-sided prayers but with discernible feedback—some seeing visions, others claiming to hear the voice of God Himself.
After countless interviews with Vineyard members reporting either isolated or on-going supernatural experiences with God, Luhrmann concluded that the practice of prayer could train a person to hear God’s voice—to use their mind differently and focus on God’s voice until it became clear. A subsequent experiment conducted between people who were and weren’t practiced in prayer further confirmed and illuminated her conclusion. For those who have trained themselves on their inner experiences, she found, God is experienced in their brains as an actual personal social relationship: His voice was identified, and felt to be real and interactive.
There’s in fact at least one book about this. From one of the reviews: