I would totally recommend "I'm a Big Sister."
I just looked and found you can get it on Amazon for $5. (http://www.amazon.com/Im-Big-Sister-Parragon-Publishing/d...)
It's still my daughters favorite book and her brother is 1 1/2 now. I would change some of the words when I read it to her so that it was more personal... like names or a specific word we use rather than the one in the book. It basically it empahsises how many cool things big sisters can do... to include being a big helper. :)
We started reading it to her when I was pregnant with her little brother. She actually loves it so much that she started referring to herself as "Big Sister." Seriously... her name became Big Sister. For awhile she would correct us when we called her by name, "I'm Big Sister!" LOL :)
Good luck with your pregnancy... and enjoy this time. I remember how sweet it was explaining to my daughter every day how special she is to our family, to her little brother (to be), and to us as our sweet little girl and our BIG helper. She really seemed to grasp the amazing role she plays in our family, how important it is to be a big sister. Of course that didn't entirely mean she was above that whole green-eyed-moster thing. But when we have those episodes we remind her of how special she is as the Big Sister. :)
Blessings :)