Why doesn't the Holy Bible contain only the original First five books of the Bible and nothing else? As I understand the Bible to state that nothing shall be removed and nothing was to be added.
The prohibition against adding and subtracting applies only to the mitzvot (commandments). The Torah itself informs us that prophecy would continue after Moses and gives us commandments on how to recognize a true prophet (Deut. 18:15-22). Inclusion in the canon (the only books we would consider Biblical) is a recognition that the work was written under some degree of prophetic inspiration and that it has a general application to the Jewish people. That the work/prophet does not transgress the prohibition of adding or subtracting from the Torah is a prerequisite, and tradition records that a great deal of effort was placed in reconciling apparent contradiction in Ezekiel with Torah law before it was finally accepted.
I will attempt to beef up the sources as I am able.
The prohibition to add or subtract to the Bible was concerning the laws as commanded by God, not concerning the amount of books we concider holy, as per Seffer Hachinuch #454 and #455 quoting Yad HaChazaka chapter two of Mamrim. There was always an allowance for prophets to enforce those laws, as seen for instance in Deuteronomy 18:15 God will set up for you a prophet like me from among your brethren. There is never found an admonition not to canonize those prophets words for all generations, and they were in fact kept specifically for this purpose as seen in the Talmud Megila 14a. On that same page we find the single instance of a commandment which was added based on an understanding from the books of Moses.
By contrast, any man, no matter how great, who stands up and claims that through prophecy he has a new law to add to the old, or that he is subtracting a law is called a false prophet, Yad HaChazaka Yisodei HaTorah chapter 8. An allowance is given for him to to say prophecy which temporarily adds or subtracts to the original laws, but not permanently.
After entering the land of Israel under Joshua, had the Jewish people kept the commandments and not turned to idolatry, the "bible" would have consisted of only the Torah (first 5 books) and the book of Joshua (because it contained the details of the division of the land amongst the tribes). But because the Jewish people sinned, the bible included the other books. I will try to find the source for this and update my answer.
The source is the Overview to the ArtScroll edition of the Book of Joshua (ArtScroll Tanach Series). It states that "Nedarim 22b teaches that ... Had Israel not sinned, only the Torah and the Book of Joshua would have been given to them; the Book of Joshua would have been given because it contains the boundaries of the tribal portions of Eretz Yisrael - information of divine origin and is essential to the nation."
The other books, consisting mostly of the rebukings of the prophets, would have been unnecessary. However, because Israel sinned, the other books were included.