Here’s a compelling article from Jane Friedman’s blog. It’s called A Writer’s Guide to Fair Use and Permissions + Sample Permissions Letter.
In it, she shares some of her insights. Here’s one that’s particularly thought-provoking:
Unfortunately, quoting or excerpting someone else’s work falls into one of the grayest areas of copyright law. There is no legal rule stipulating what quantity is OK to use without seeking permission from the owner or creator of the material. Major legal battles have been fought over this question, but there is still no black-and-white rule.
Have a look at the complete article for more of Jane Friedman’s inputs.