Actually O'Reilly is a publishing company that is the definitive source for books on UNIX. Their book covers are unique and easy to spot: a white background with an accent color, and a line drawing of a different animal -- except for UNIX Power Tools which has an electric drill.
If you plan to tackle any particular utility in UNIX, for example, uucp or awk, it's worth the time and money to purchase the appropriate O'Reilly book on the topic.
You can order books either by visiting your local computer book store or by sending e-mail to email@example.com. If you don't know what to order you can get information about O'Reilly books from their Web page.