Which book will you suggest to understand major religions of world like Hinduism, Christianity, Islam, Buddhism, Taoism, etc?

if you are yourself not having some degree on sociology, history or linguistics or at least some personal study on these subjects for a long enough period, please refrain from directly reading religious texts. Without the knowledge of the context of these texts and the technical knowhow of what means what, you will not understand anything. So I shall suggest that you read the works of well known Subject Matter Experts, university professors, researchers and authors whose writings are objective and have been published by well-known publishing houses like Springer, Oxford University Press, Cambridge University Press etc.

Now, I shall come to your first query. I would suggest a few interesting books which might give you an understanding of some of the world religions. But before reading any of these, please read Yuval Noah Harrari’s work “Sapiens- A Brief History of Humankind”. This would make a structure in your mind into which you can fit, the books you read later, as modules. So here are some of my personal suggestions:-

African Traditional Religions:-

African Traditional Religion by Aloysius Lugira
Native American Traditional Religions:-

Native American Religions by Paula Hartz
Austronesian Traditional Religions:-

Ancient Religions of the Austronesian World by Julian Baldick
Australian Traditional Religions:-

The Firm and the Formless: Religion and Identity in Aboriginal Australia by Hans Mol
Zoroastrianism:-

Zoroastrians: Their Religious Beliefs and Practices by Mary Boyce
Buddhism;-

Buddha and his Dhamma by Dr. Ambedkar
Revolution and Counter Revolution in Ancient India by Dr. Ambedkar
Haunting the Buddha by Robert DeCaroli
Vaishnava Sahajiya Cult of Bengal and Panchasakha Cult of Orissa by Paritosh Das
Christianity:-

The Lost History of Christianity by John Philip Jenkins
Sikhism:-

A History of the Sikhs Vol. I by Khuswant Singh
Islam:-

Why I am Not a Muslim by Ibn Warraq
The Lost History of Christianity by John Philip Jenkins
In Search of Shiva: A Study of Folk Religious Practices in Pakistan by Haroon Khalid
Islam o Bangali Musolman Somaj ( Eng. Islam and the Bengali Muslim Society) by Asim Ray
A Brief History of Indonesia by Tim Hannigan
Hinduism:-

Lokutsob o Lokdebota Prosongo (Eng. On Folk Festivals and Folk Gods) by Barun Chakroborty
Banglar Tantra (Eng. Bangla’s Tantra) by Pachkori Bandopadhyay
Zoroastrians: Their Religious Beliefs and Practices by Mary Boyce
Debloker Jounojibon (Eng. Sex Life in the World of the Gods) by Atul Sur
Which One of Us are Aryans by Romila Thapar
Time as a Metaphor of History by Romila Thapar
Revolution and Counter-Revolution in Ancient India by Dr. Ambedkar
Castes in India by Dr. Ambedkar
Who were the Sudras by Dr. Ambedkar
The Untouchables: Who Were They and Why They Became Untouchables by Dr. Ambedkar
Gouropodacharjo Krito Agom Sastro ba Buddhottor Bedanto (Eng. Gouropodacharya’s Agom Sastro or Post-Buddhist Bedanto) by Silachar Sastri
In Search of Shiva: A Study of Folk Religious Practices in Pakistan by Haroon Khalid
Globalism:-

Globalization: A Very Short Introduction by Manfred Steger
Nationalism:-

Nationalism: A Very Short Introduction by Steven Grosby
Democracy:-

Democracy: A Very Short Introduction by Bernard Crick
Fascism:-

Mussolini’s Intellectuals by A. James Gregor