In 'Peter's Rock in Mohammed's Flood, from St. Gregory the Great to St. Leo III,' T. W. Allies delves into the historical and theological relationship between Christianity and Islam during the early medieval period. Allies employs a scholarly approach, citing primary sources and theological texts to explore the interactions and conflicts between these two religions. The book provides a comprehensive analysis of the political and religious landscape of the time, shedding light on the complexities of religious dialogue and conflict in the midst of a changing world. Allies' writing style is dense but informative, making it a valuable resource for students of medieval history and religious studies alike.