I am taking a good hard stab at a prologue for my novel Viral Times this week. In the process I've discovered how few writing books address the nuances of this pseudo-beginning for a story.
Revision and Self Editing has a two-page section called "The Use and Abuse of Prologues." Good stuff. I found the advice in Manuscript Makeover even more helpful. "Some agents refuse to read manuscripts with prologues," Elizabeth Lyon warns, but the section also explains in significant detail how you can avoid undermining yourself by using a prologue. Also, Beginnings, Middles and Ends has good instruction on the subject.
In summary, a prologue has its mission: Tell parts of the story the reader wants to know before the main story commences. Set a tone with the best language you can craft. Raise questions, too, so readers are motivated to continue.