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. When she is ready for those books, she will get them.
I think most just want their baby to be welcomed with joy and excitement just like any other baby. I suggest you do whatever you would normally do and maybe just alittle extra. A really cute outfit (Disney of course

) and then maybe something that would be good for the baby but not received in a negative way. Colorful and tactile toys are a good one at that age, they will want to encourage reaching, grasping etc.
And be there to visit, hold the baby, talk to the baby, tell her how beautiful the baby is. Many friends will back away, many will send a gift but not visit. If you are comfortable and loving towards her baby, that will be the biggest gift of all. Your comfort may also give her the opportunity to talk about the baby, her fears, her plans etc. I remember having family members that I couldn't even talk to about that stuff and it hurts. I know she is a co-worker but you may be just far enough removed that you could be someone she could talk to alittle bit at the beginning until all the family jumps in with their support.