The basic text for the study of God, of course, is the Bible, our only inerrant source of truth. Interpretations of Scripture in commentaries and theological formulations will vary, so it is helpful to compare notes. We can learn much from each other, especially in working through differing points of view.
In my own study, I have perused the writings of the early Christian fathers, then followed developing thought through church history, giving particular attention to the great reformers, while not forgetting voices of renewal from the Moravians, Pietists, Puritans, and Wesleyans. Many scholars of more recent vintage have also been consulted in an effort to understand the theological underpinnings of the Gospel.
Giving my research more practical input, countless sermons also have been reviewed from preachers with a passion for souls. I have often told my students, the best way to understand theology is to see how it preaches. Across the years, too, a number of books described as theologies of evangelism have appeared, one of the most recent being the work of Lewis A. Drummond. While I am appreciative of these works, my own study takes a more comprehensive approach and gives greater attention to application.
― Robert E. Coleman, The Master Plan of Evangelism