You can't observe everything yourself. You have to seek out sources with minimal bias or understood bias and place some amount of trust in them, or you'll be paralyzed.

And it's not like autobiographies in general are to be trusted either.

You gave an extremely narrow example. There are a multitude of resources on virtually any subject. Would I expect a textbook on African American history to cover Malcolm X with enough depth to be considered a definitive source? Of course not, but that doesn't make the textbook useless. We don't have to limit ourselves to a single source of information, so why would we? The original post I was replying to said "How could you have formed an opinion if you haven't read the book." As if reading that particular book is the only way to learn anything about the man. Malcom X's biography is probably a great book, and I hope to find the time to read it soon, but that doesn't mean the statement wasn't absurd.