The book is a transcription of old handwritten church registers from the mining towns of Silver City and DeLamar up in the Owyhee Mountains. Albertsons Library Special Collections holds the original church registers, which are now fragile and written in script that is often difficult to decipher.
Old church records like these are important because the State of Idaho did not begin compiling birth and death records until 1911. So before that date, church records (with baptisms, burials, etc.) are often the only vital records there are.
Jones, Patricia Dewey. (2006). "Early Records of the Episcopal Church in Southwestern Idaho, 1867-1916 : Silver City and DeLamar". .