I had an email recently asking regarding what's allowable in citing or being inspired by other sources and I thought you might find it useful as well:
Q: We were wondering whether it’s all right, having read books like The Secret, to add something about positive visualisation that a character does in an attempt to become what they want to be. Is it all right to add this- what someone else has written about - and then acknowledge it in the Acknowledgements part of the novel?
Yes, you certainly can have your character using The Secret and as long as you don't use too long an exact quote you don't even need to acknowledge it separately. It's no different from, for example, having a character eating a Mars bar or referring to them watching a particular TV show.
It's also OK to quote a few sentences exactly as they are written in the book. The rest of the time you might want to paraphrase. For instance, you might write something like, "That night, she read two more chapters of The Secret. The part that says if you want to be thin you shouldn't look at fat people worried her. Her three best friends all were overweight. Could this be the real reason she had gained two dress sizes in the last year? Maybe it had nothing to do with eating chocolate chip cookies every evening."
Q: What is the procedure if you write about a character that has some characteristic of yourself and some of a friend, but exaggerated? Is it all right to do this - to base things on people but change the details and exaggerate traits?
Yes, every author does this.
Q: Can you quote from the past, things like ancient proverbs in a fiction story or do you have to give exact details of where the character got it?
You don't have to give details about where they got it. No problem quoting proverbs, bits of the Bible, bits of poetry or novels that are in the public domain, etc.
Q: Can you base a character you got partly from an idea from a TV show?
Sure, as long as you change the name and it's not a 100% copy. You want to be sure that your character has several attributes that he or she doesn't share with the TV character. For instance, you might switch the gender, the age, the social status, the physical appearance, and so on.
It gets trickier if you are basing the character on a real person and make him a villain. For instance, if you wrote a book about a property developer who showed a lot of resemblance to Donald Trump and had him killing someone, you might be in for a lawsuit. One way around this is to mention the real person in passing. So let's say my developer is named Carl. I might write, "Carl modeled himself on his idol, Donald Trump, but was willing to go further than his hero to be successful. All the way to murder."
You might want to have a look at some of the tips on my page about protecting your material, because that goes into more detail about what can be copied or not. It's at http://timetowrite.com/protecting-your-material.