Note: the following information was collected by posting many questions on forums for small publishers; in many cases it represents opinions and should not be taken as competent legal advice.
As a compiler of a book of quotations, I sometimes get questions like, "How do I get permission to quote someone in a book?" or "What do the copyright laws say about quoting someone?"
Here are a few things I learned while researching the subject for myself.
To start, if you want to quote someone, you need to get permission from the author or publisher. Consider it to be a necessary step in writing your book.
How do you go about doing it? I sometimes had success with sending the author/publisher a faxed agreement and having them fax back a signed copy. Other times they preferred to just send a quick email saying it's ok, as long as I agreed to provide a "by-line" in the back of the book.
Let me say this: it is hard getting "permissions". If you only have 1 or 2 to go after, it isn't too tough. But in my case, with a book of quotes, wow...it was difficult.
Copyright law in this area is called "Fair Use". Fair Use says that you can copy someone without permission if it's for educational purposes or if they've been dead for over 50 years. Keep in mind that these two points aren't exactly 100% fool-proof. You can still get in trouble.
In my case, at a late stage in the publishing process, I decided to re-compile my whole book using quotes from people all dead over 50 years. This was a bummer, since I had my heart set on a few really good quotations. Why did I re-compile the whole book? Because it just became way too hard to track down so many "live" authors.
Some people gave me permission to quote them, but other people flat-out said "NO!"
Before you can ask for permission to quote someone, first you need to find out who they are! How do you find out when they lived? When did they pass away? Google is great for this...especially if you type their names in quotes: ex. "John Doe".
If you have a lot of people to track down, there are services that can do it...but I never found anyone who offered such a service. I called a few BIG publishers who said they have small departments that do this job. It's tricky, so they themselves like to stick to long-dead people to quote.
Apparently, a lot of people do quote other people without permission. Will the copyright owners find out? Will they care? Will they be honored to be included in your work? Will they look at it as promotion or damage?
What if you can't locate the owner and you really "need" to use that quote? When/if the copyright owners find out, will they take legal action against you for a small quote? And, what if you provide a generous "by-line" in the "Resources" section at the back of your book? Under Fair Use laws, they have to prove that your quote caused them financial damage. If it appears to have actually helped them, by referring people to their book/company/service/website, is that damaging? You decide.
Also, it appears that the courts will take into consideration how big your quote is in relation to the work as a whole. If it's a small quote in a big book, some people say you're "safer".
If you can't get permission, maybe you can find another similar quote, or perhaps you could reproduce/re-write the idea into your own words...without blatantly plagiarizing. Your last line of defense would be writers/publishers insurance...which can or can't be expensive. Shop around. Spannet.org has some specials if you join their association.
As one last thing to think about: please don't take my words as "Gospel" or competent legal advice. Check out publaw.com and ivanhoffman.com for some very good information on copyright law.