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.
A Mere Christianity, by CS Lewis. Imitation Of Christ, by Thomas A. Kempis.
... The Bible
One, the "Bible".
The Holy Bible
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.
Judaism, Christianity, and Islam have books considered holy scriptures. For example, Judaism has the Torah, Christianity has the Bible, and Islam has the Quran.
Christianity, as a whole, has only one 'Holy Book' and that is the Bible. There are various versions of it in translation around the world.