I am considering starting to sell lineart (or try to

OK, I thought of this question after seeing someone else's post. I have always assumed that I could not use photos out of books or magazines as reference for lineart because it has to be under copyright of some sort, yes? Am I wrong about this? Can we use photos from books as reference if we own the books? I have strayed away from doing this because I thought it would not be acceptable, but since I have seen some others doing it now I am unsure... and with all of the books and magazines that I have things would be so much easier if I was able to use those. (I am talking about reference for poses or colors if that helps answer the question in any way).
I have a feeling I have had it right the first time, only using stock images, but can someone who knows for sure please answer this? If I am able to use books and so on I would be able to get a lot more done a lot faster... but since I AM considering SELLING the pieces I REALLY need a definite answer on this, no "maybes" or "I think sos" because I don't want to end up in any trouble.
(if it's an OP that answers I would be happy to give a link to the posts that lead me to ask about this, I am not comfortable pointing anything out otherwise though in case it would be putting a strange spotlight on the people who posted).
Thanks to anyone who can point me in the right direction on this.
