If it's only a short passage, then that's allowable anyway.
Fair use principles generally relate to the amount of the original work used (proportionally) and the purpose of use. For-profit usage where a large portion of the work is copied falls on the probable violation side; small excerpt with no profit motive is likely to be acceptable.
The other thing to remember is that the ideas themselves are not copyrighted. It's only the text that is being used to set them down, that is copyrighted. So if you express the ideas in your own words, you don't have a problem.
Putting ideas in your own words is _usually_ safe --- but, again, it's a matter of proportion. Taking a book and rewriting it in your own words is still a probable copyright violation; the concepts are copyrighted as part of the critical creative elements of the story. It's not actually just the literal words.
Finally, even if you may be given approval in a court case (and copyright violation cases are very complex because the rules are highly interpretive), the legal proceedings could still be difficult.
Every single copyright situation is unique, since it's a set of laws governing the rights of use on creative work --- the parameters for use and work are highly variable, and interpretation is a key element of the laws.
Edited by joedolson, 12 May 2008 - 02:48 PM.