I am not making a legal argument, just making an observation.
The left has been making outlandish statements about Republican candidates with impunity for years, ie: October Surprises. I think this is one person (Palin)that may take them to task and actually not roll over as so many have in the past.
Only if she's not running. She will not want the aggravation, time sink, and expense of dealing with a lawsuit while she's running for President.
Not to mention discovery. I wouldn't be surprised if "authors" frequently come up with outlandish lies abut public figures just to provoke a lawsuit so they can get into the discovery phase and go digging for juicier gossip.