I did the same with WorkTips and HomeTips cover images from these book order cards. I created postcards and then mailed one to everyone in my database. That generated a number of orders. Mitchell Gooze, CSP Both have their place. Used postcards to announce book signings etc.
Used book covers for other, more limited in size, promotions.
Be aware that with your contact info on the reverse of the cards you will have a two sided job, but it won't add much to the cost. With all the pieces on one set of negatives, you'll probably offset the cost of the second side anyway. A large part of your printing costs will be in prepress and set-up, so the more you can get on the page the better. I usually print 2000 covers at a time. That allows me some economies, but lets me change things later.