I voted Sears book. If she is open-minded, she may just change her whole outlook because of the book. You don't know what her ideas are now about raising kids, right? This could have a huge impact. I'd go for that impact above the usefulness issues of say, slings and CDs, which if she is going to be mainstream-style parenting she wouldn't get much use out of anyway.
If you have extra $ to throw around I'd get a mothering mag. subscription. OR, I'd get the subscription and hand-me-down her a couple of your old books, a sling, etc if you can spare them (I wouldn't do the hand-me-down as a shower gift, but I would do it at some other time, maybe before or after the shower).