1. Tell the author you couldn't put her book down.
2. Tell the author you read it in one sitting.
3. Tell the author you stayed up wayyy past your bedtime to read it.
4. Tell the author you can't wait until her next book comes out.
5. Quote a line from the book back to the author.
6. Tell the author you were late getting back to work from your lunch hour because you were reading her book.
Things that haven't happened to me, but that would really, really make my day (and might even score the person an Advance Review Copy of the next book....):
1. Give the author chocolate.
2. Send the author flowers.
3. If you also are an author, offer a great quote
4. Offer the author a night with Johnny Depp.
(Or Clive Owen.)
(Or Gerard Butler.)
Jus' sayin'.