The front side of the card, with the cover and a short link to the book on Amazon.
The back side of the card, with a brief synopsis of the plot.
I recently made up some business cards to help a recent author spread the word about his new ebook. The author was very appreciative:
WOW! And a big thank you for those great looking cards. It’s obvious you put some thought to the cards design and Rose and I are thrilled with them. . . .
The cards look great and I can’t thank you enough. Now I have a mission! Get those cards out!