One, the "Bible".
A Mere Christianity, by CS Lewis. Imitation Of Christ, by Thomas A. Kempis.
There are many different books on Christianity and you would need to specify what topic under Christianity you would like to research to narrow the choices. The Bible is one of the best sources because it is the Christian religious book.
The scriptures of Christianity are contained in the Bible: 39 books of the Old Testament and 27 books of the New Testament.
The Holy Bible
... The Bible
There are 39 books in the Old Testament and there ar 27 books in the New Testament. That makes 66 books in total. However, there are variations depending on which branch of Christianity or Judaism that one belongs to.
The Old Testament & the Torah are the same Books. Abraham is the Father of both Judaism & Christianity.
Prayer
Desiderius Eurasmus was a theologian, he wrote books on religion, specificly that of Christianity.
The country that bans and confiscates books on Christianity is Uzbekistan.They impose a strict censorship on all religious material that comes into the country.
Globally speaking, the Holy Bible.
C. S. Lewis has written seven books in "The Chronicles of Narnia" series. Additionally, he has written multiple other fiction and non-fiction books that draw on Christian themes and mythology, such as "The Screwtape Letters" and "Mere Christianity".