A companion book provides elaboration on another book or general topic. They cover themes and concepts within a specific subject.
Oxford Very Short Introductions
Very short books giving an overview of religions, their developments and important issues. Useful for general overviews on the topic
• Christianity: A Very Short Introduction
• Islam: A Very Short Introduction
• Ritual: A Very Short Introduction
• African Religions: A Very Short Introduction
• Tibetan Buddhism: A Very Short Introduction
And many more...