I think this is a lovely idea, and I posted my suggestions earlier. Upon reflection, however, I’m not sure how well the actual implementation would be received. Generally, unless they’re titles/authors I’ve specifically requested, books I’ve received tend to leave me feeling less than enthusiastic. They may be very nice books that I might even enjoy reading, but they tend not to be the books I get excited about.
As an alternative, maybe you could give her money/coupons representing money towards a shopping spree at the bookstore. If you give her $1 on day one, $2 on day two . . . $12 on day twelve, the total is $78.
Another alternative you might want consider would be to mix up the selected novels with other bookish type gifts. You could intersperse novels with things like a booklight, bookends, a magazine subscription, a puzzlebook, a coffee table book related to other interests, etc. You might even throw in some treats to indulge in while reading (hot cocoa, Japanese sodas, bottled coffees, chocolates, cookies, etc.)