Fair use or fair dealing allows limited reproduction of copyrighted works for specific purposes, such as study or criticism.
1. The source of the material must be acknowledged.
2. The amount of the original copied.
Generally speaking the less, the better. "Quote or closely paraphrase as little as possible to make your point: under 10% is best; above 20% is high risk...although in some cases 1% may not be fair use, and 100% can be fair use, it?s impossible to provide percentage guidelines that always work".[1]
3. A small paragraph or sentence paraphrased or quoted verbatim from a source is likely fine, quoting or paraphrasing an entire article from a source is likely not.
4. The purpose of the copying.
If you reproduce some of an article or text as a guide or recommendation to their books, that is more likely to be considered a valid purpose than if you create content which could serve as alternative but be in a real sense a plagerism.
basically... nothing is 100% certain, and if in doubt.... as I learn... ask. For example, every video and every site that goes on Past Horizons, they get an email asking for permission... its easier that way (of course some emails go astray) Most people will be fine, but I do get the occasional who says, no... I don't want it on... in which case it comes down. You may also have the fun, of getting permission from people, who have already lifted content from elsewhere. <sigh> All you can do is try your best to get permissions... and be ready when it goes wrong (as it will). Do nothing knowingly..
[1] Howard Zaharof. Copyright: A Writer's Guide to Fair Use in Copyright Law. Writer's Digest.
http://www.todays-woman.net/article1528.html
"No job worth doing was ever done on time or under budget.."
Khufu