As the parent of a child (

now 24) with Down Syndrome and having been involved with groups who do initial contacts with new parents...........my advice would be to treat this baby as if you would any other.
I appreciate you wanting to the do whatever you can for your friend. While I had no adjustment issues when he was born I saw many parents who did. Receiving a "special" book may not necessarily be well received. It may be interpreted as a consolation prize.
I have to agree with you! Our daughter was born at 22 5/7 weeks (~15 oz at birth but went down to 13 oz). We didn't get cards, calls, presents, etc., until she had been home from the hospital for a long time. People didn't call to see how she was doing or how we were holding up. We didn't know if she'd live for months. I'd really have appreciated congratulations, a friend, baby presents, and, well, dinner. Doesn't every mother of a newborn need meals?
I remember being terribly offended by a comment made by a photographer friend when my daughter was about eight months old. I asked about her taking pictures of my daughter and she said it'd be better to wait until she got a little bigger. I was totally offended. Now, this offense was totally misguided but I wasn't ready to see my daughter as different from other kids. She was 8 months old and weighed 8 lbs. Of course she was different. Of course it would have been difficult to photograph her. But I wasn't ready. I wanted to celebrate our child.
So, I'd recommend giving your friends a baby gift with congratulations about the birth of their wonderful new blessing. Then, I'd ask what your friend wants/needs regularly. Having a child with a disability can be terribly isolating. We lost all of our friends. My hubby and I have been out together alone maybe 4 or 5 times in ten years.
You know, we have a great kiddo. But, it can be tough to take it all in at once. We were fortunate because, while our daughter was gravely ill, we had a long time to discover what her abilities would be. This family, while they know that their child will have issues, won't really know his abilities for quite a while.
Sorry, long winded today. Please be an available friend for this family.