I think the policy that NOLO has is good. I've included the portions I believe most important points are the 3 included below. Read the whole article, however, it is quite good.
Subject to some general limitations discussed later in this article,
the following types of uses are usually deemed fair uses:
- Criticism and comment -- for example, quoting or excerpting a work in a review or criticism for purposes of illustration or comment.
- Research and scholarship -- for example, quoting a short passage in a scholarly, scientific, or technical work for illustration or
clarification of the author's observations.
- Nonprofit educational uses -- for example, photocopying of limited portions of written works by teachers for classroom use.
Also of importance is that we are not trying to distract from the original works. No one is going to not read a book because a 1 sentence quote was posted from it, because they feel like the book is complete. Posting a whole chapter, however, would probably not be wise.