The holy book of a particular religion. You pick which holy book you wish for whichever religion or deity. The +1 to Religion bonus only occurs once obtaining a holy book, and is not repeated for each additional holy book you possess.
A book which describes the religious views of the nation where you bought this book. +1 to Insight.
A book which describes the religious views of the political region where you bought this book. +1 to History.
A book which describes the religious views of every known race. +1 to Religion.