Since we recently had the discussion on DRM-protected e-books and whether or not it is OK for you to use cracking tools to remove the protection, I found this very interesting and relevant page at Standford University:
Measuring Fair Use: The Four Factors
The article discusses four factors whether a particular use is a fair use or not:
1. the purpose and character of your use
2. the nature of the copyrighted work
3. the amount and substantiality of the portion taken, and
4. the effect of the use upon the potential market.
Great read and it should help some concerned minds to decide what to do about their private use of e-books.