With impeccable scholarship for specialists as well as a lucid and accessible introduction for the wider public. (artbook.com)
Islam: Faith and History, by Mahmoud M. Ayoub (Oneworld), is an excellent and very accessible introduction. (religion-online.org)
The more recent Signs and Wonders: Why Pentecostalism is the World's Fastest Growing Faith by Paul Alexander is a more accessible introduction. (rachelheldevans.com)