Although I preach less-is-more and all-you-need-is-love I do realize you likely do not wish to show up to showers and so forth empty handed ...
I am not one to always follow this advice, and do have a soft spot those impractical items that all of us cannot resist (newborn knit bunny hats!), but this is what new mommies really want and NEED:
1. 4th Trimester / Postpartum Items :
Basically, anything from the lady section in the grocery/pharmacy...
'Recovery' Bath Soaks and Salts
Earth Mama Angel Baby products
Mama Teas (milk and uterus)
Cute hot/cold packs
If you know her well:
Real underwear, nursing bras, nightgowns, a robe!
Pretty light scarves as nursing covers, stroller covers etc.
2. Thank You Note Set
Small extra special cards that speak for themselves so mama doesn't have to write much.
If you had any go-to's, these will save her soul from the wrath of the internet!!
Magical Beginnings, Enchanted Lives Deepak Chopra
The Womanly Art of Breastfeeding or similar
Secrets of the Baby Whisperer Tracy Hogg
Bringing up Bébé Pamela Druckerman
The Nourishing Traditions Book of Baby and Childcare Morell and Cowan
Your favorite baby books are also always appreciated and usually loved for a very long time ...
Bring a little something for an older child or hand your gift to that child for 'their baby'.