If Amos, Nahum, Habakkuk, Haggai, Malachi were names of real people, why are they not found elsewhere in Biblical history?

You are asking the wrong question, in my opinion.  Here is the correct question to ask:  Are Amos, Nahum, Habakkuk, Haggai and Malachi real people?  It is not the job of a historian or a biblical scholar to explain why certain people are not mentioned more times than others.  Some people in the Bible are mentioned dozens of times, some people are mentioned a few times, some are mentioned only a couple of times (Jonah, for example), and others are only mentioned once.  The fact that a person is only mentioned once is not evidence that they did not exist!!!  This is a nonsense argument.

So, back to the question.  Is Amos a real person?  The answer is that he is the claimed author of the book we know of as Amos.  Is he a real person?  The proper answer is that we are not absolutely sure.  Probably he is.  In fact, most likely he is a real person.  Is there any reason to believe that he is NOT a real person?  Does the Muslim making this argument (and I am assuming that this criticism is coming from a Muslim source), have evidence that they are not real people?  What would be the argument?  Apparently, the author was famous enough among the Jews to get some of his writings into the Old Testament.  Is it possible that the book we call Amos was read by the Jews, but his name was forgotten over time?  Is it possible that the author of Amos was not, in fact, named Amos?  Sure, that is possible.  It is not possible to present concrete an unambiguous evidence for such a thing.  What would be the big deal if the name which came down to us was not the actual name?  The  only REAL question for a student of the Bible is whether Amos is inspired by God, not the name of the author.  However, I believe that common sense tells us that very likely the actual author of Amos was a prophet named Amos.  The same goes for Nahum, Habakkuk and Haggai.  Nahum means comfort, and he is described as being from Elko. Habakkuk means embraces. Haggai means holiday.

The case with Malachi is somewhat different.  The reason is that the name Malachi means “messenger of God.”  Unlike the other four books, one can make a fairly reasonable argument that the book of Malachi was anonymous, but that the Jews gave the author the title “messenger of God,” because that is what he was. Therefore, it is possible that there was no person named Malachi. I am not saying that this is the case, but I am acknowledging that it is a significant possibility. Again, all that really matters is that the book we call Malachi is inspired by God, which I am convinced it is, partially because of the amazing prophecies in it, especially about John the Baptist.

My conclusion is that for Amos, Nahum, Habakkuk and Haggai, it is very likely that these are the actual names of the actual authors of these four books.  With Malachi it is somewhat likely that this is the name of the author of the book, but, unlike the other four, there is a perfectly reasonable theory that the author was not named Malachi.  But of course, all that really matter for all five of these books is whether they were in fact inspired prophets, speaking for God.  Given all the prophecies contained in Amos, Haggai and Malachi, there are clear marks of inspiration. It is not a big stretch at all to believe that all the books God caused to enter the Hebrew canon are inspired by God. The name of the author is simply not an important issue for these books, but, like I said, common sense says that it is very highly likely that for the first four the names we have received are the correct names of the authors.

John Oakes