So how do we decide what to document? The standard rule is "You do not have to document common knowledge on a topic." That means information that most educated people know, or people in your field. You might need to look up information on who won the Pulitzer Prize in 1970, or who won the World Series in 2004, but that's information anyone can know.
You also do research. If you feel your information comes out of your blending together information but not specifically quoting from anyone, you're probably all right. While working on this series, I have seven books on my desk and I've checked three trusted online sources. But I haven't quoted from any of them.
For example, I'm currently working on a book with Bobby Davis, a highly successful pastor in Tennessee. I've been working and writing a chapter on a specific biblical topic. I can go beyond common knowledge because of my educational background (I'm a seminary graduate) and my experience (I was a pastor for 14 years). I can't possibly remember where I learned the information, so I don't try to cite it. However, when I refer to the Bible, I document by quoting the verses followed by chapter and verse and the translation.