You've been assigned a research paper on the holy wars. which of the following would provide a factual base for  additionalresearch?  

You did not put options in your question, but the word you are looking for is ENCYCLOPEDIA. 
An encyclopedia is a book which gives information on many subject or on many aspects of one subject and it is usually arranged in alphabetical order. An encyclopedia will be a rich source from which one can gather factual information about holy wars. All that one needs to write a comprehensive essay on this type of war can easily be found in the encyclopedia.