There is something about writing and having a point of view that can be a challenge. The thing about sources is that you will have to think about where and why these were published. This holds all the more true if you are planning to make money with your writing.
If there is any point which is most important it is the fact that as a writer, you must check your sources. Then you must double check.
This goes for writing in both fiction and non-fiction. This is not easy. For example, you might be writing a science fiction piece with intent for it to be published by a traditional publisher, but you have written a piece about a technology which is just gaining popularity right now, you will need to know at least a bit about it.
In non-fiction, not knowing much about your sources can be even more dangerous. You have probably received some money for your proposal or something of the sort and you find some information about the subject. One thing you need to do is look, and really read the source and ask questions, as if you do not, you can be very wrong, and the consequences can be in terms of money and future chances of publishing a successful book.
This is why checking your sources and knowing the facts is a money thing. You can make money for being a writer with trusted sources or you can be a writer who loses money because you did not check your sources and question them.