What is "hagarism"

Hagarism: The Making of the Islamic World is a 1977 book about the early history of Islam by the historians Patricia Crone and Michael Cook. Drawing on archaeological evidence and contemporary documents in Arabic, Armenian, Coptic, Greek, Hebrew, Aramaic, Latin and Syriac, Crone and Cook depict an early Islam based on Christian and Jewish origins different from traditional mythology. The book has been hailed as a controversial but seminal work in its branch of Islamic historiography.