There is only one Holy Book in Christianity and that is the Bible, however, Protestant Christian denominations use a bible with 66 books, while Catholics use a bible with 73 books. The Bible is considered to be "the word of God" by both Protestant Christian groups and Catholic groups.
The reason for the difference in the books of the Bible can be traced back to the early Christians. The early Christians held at least five councils where they attempted to decide on the canon of the Bible, however, it was not until the 4th century at the Second Council of Carthage that the Christians were able to decide on a Bible. They decided upon the Bible that the Catholic Church continues to use today. This Bible had 73 books, including 46 in the Old Testament and 27 in the New Testament.
More than 1,100 years later, Martin Luther led the Protestant Reformation and wanted to drop seven books from the Bible for his own beliefs and purposes. He did so and the Protestant churches have followed this practice ever since. Catholics, however, kept using all 73 books.
The Bible is the holy book and though all Christians interpret the Bible differently, they all believe that it is a book that has been inspired by God and therefore is his word. From here, some interpret the Bible as literal, while others interpret it in parable form among other interpretations.