Firebug is a novel written by Robert Bloch. It was first published in 1961.
Description[]
His name is Philip Dempster. Investigating a number of phony "churches," Dempster becomes caught up in a web of intrigue, arson and murder. Shortly after he visits each tabernacle, the building goes up in flame -- often with the preacher inside. Is Dempster the firebug or an innocent victim? He must learn the truth before his sanity crumbles to white hot ash.