F.F.
Hi S.,
I am 15 weeks pregnant and I'm still nursing my son who just turned 2. I second Lisa's recommendation of "Adventures in Tandem Nursing" by Hilary Flower, and if you decide that you need to wean or at least night wean, then I recommend "How Weaning Happens" by Diane Bengson. She has some good advice on how to gently lead your child toward weaning without ending up in a battle of wills.
I also recommend going to www.mothering.com to find your due date club. You will find lots of women there who are due the same month as you and many of them will still be breastfeeding too. If you find yourself wanting in-person support, you could attend the North Central La Leche League meeting which is on the second Tuesday of every month at Central Market on Broadway. There are several women in our group who are nursing toddlers and/or are pregnant while nursing.
Finally, (sorry to write such a long response) I second what Lisa said about midwifery care. I had an OB for my first birth and I'm seeing a midwife this time, and it is such a difference in the level of care I'm receiving. I feel like a partner in my birth, my midwife spends time with me, and she's teaching me cool stuff. If you've never really considered it but are curious, you should watch "The Business of Being Born" and/or read "Ina May's Guide to Childbirth." There is a really nice Birth Center here in the medical center area, and there are lots of midwives who do homebirths.
Good luck with your pregnancy and with making things work with your daughter whatever you decide!