Why use Encyclopedias?
Encyclopedias provide an overview and introduction to a topic. Encyclopedias are meant to get you started on a topic and can help you become familar with issues and terminology in a subject area. They will provide you with background information and additional references. They are not research documents.
An Illustrated History of Islam by 
Encyclopedia of Islam and the Muslim World by 
The Princeton Encyclopedia of Islamic Political Thought by 
The Oxford Encyclopedia of Islam and Women by