Why is bible called the group of books?

Each part of The Bible, each "chapter" so to speak, is called a "book". The reason is historical: the Bible is a collection of different writings, that were more or less independent from one another. Somebody decided that those writings were to be considered Holy Scripture. In fact, opinions on what is included in the Bible vary - some of the books are recognized by some Christians (and other religions), but not by others.