Many years ago when my brother was in middle school he shared a lesson that his teacher had just taught about the Mormons. They were studying 19th century American history and she decided to include religious groups in that history. So he retold how she taught about Joseph Smith's martyrdom. This was a story that included exciting chases on horseback, hangings and more. Very exciting...but none of it was true.
Now this story was not in his state issued history book so I'm not sure where she came up with it. (Mind you this is long before the days of the Internet.) Her story was similar to one that I have read in a 19th century "autobiography" that was done by an author who reportedly left "Mormonism." These 19th century autobiographical novels/anti-Mormon literature were very dramatic, often plagiarized and included some fuzzy history. Any source, no matter what it is, needs to be evaluated for potential problems.
So I ended up writing a note to his teacher explaining that her story, while exciting, was not historically accurate. I then provided the real story about the death of Joseph Smith with sources. My brother gave it to her the next day and she quickly replied that my version was just one story of how Smith died. At the time it was a good lesson for me that some people, even when given historically accurate evidence will deny that information.
Now, Joseph Smith was a historical figure who can be studied. People witnessed his death, both those who killed him and those who were Church members. While there might be different versions of that core story, the "facts" remain the same.
Fast forward to present day. I'm reading a book the other night and it talks about the prophet Joseph Smith's time in Utah. So I reread the same sentence about five times to make sure I'm reading it right. (Right about now anyone who knows about the Mormon church should be scratching their heads). This was a book that I had been enjoying. It had some interesting historical "facts" that I felt could be useful to genealogists up until that point. I found myself perplexed over how a book with historical "facts" could have got something so wrong.
I am telling these two instances as a cautionary tale. First, as we have all heard don't believe everything you read. This is not just a pithy saying. Anyone can write a book. If the person hasn't cited any sources than what they have written is hearsay. If you want to use it for your genealogy, then you need to verify the information and not take it at face value, even if it is something that the author says they witnessed. It's sorta like what one of my friends use to tell people, do you want to research your Johnson family or someone else's? If you take everything you read at face value you might find out 5 years down the road that you researched someone else's family.
So while it is nice to believe that all your genealogy, back to the olden days, was done by someone else and they posted it on the Internet for you, if there are no sources then it is nothing but words on a page. Use it as a clue but do your own research to verify, no matter how good it looks or who the person was. This is a lesson many genealogists know. But in addition, no matter what type of book you found some information that is useful to your research, I don't care who the author is, if it isn't cited, do your own research to verify it.