Whenever you decide to directly quote, excerpt, or reproduce someone else’s work in your own—whether that’s a book, blog, magazine article, or something else—you have to consider, for each use, whether or not it’s necessary to seek explicit, legal permission from the work’s creator or owner.
Unfortunately, quoting or excerpting someone else’s work falls into one of the grayest areas of copyright law. There is no legalrule 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.
However, probably the biggest “rule” that you’ll find—if you’re searching online or asking around—is: “Ask explicit permission for everything beyond X.”