I wouldn't give them something obvious about giving up smoking (such as inspirational books or some such.) Just give them something else they'd like. Do they have hobbies? Do they like to read? Do they like getting pedicures?

I like the pedicure idea--or something else that's a pampering, special way to take care of oneself (since the person is taking care of him/herself in a healthy way by quitting smoking), like a massage, basket with bath products--or something like a nice pair of earrings, a trinket box, or a little framed print.