I'm headed in this direction myself.
Two books that I have, but have not yet read are:Mothering the New Mother
and Nurturing the New Family
I was hoping to take a course offered in my area too but I just can't afford it right now for time and financial reasons. At this point, from what I've heard from other PP doulas, is that deep listening is key, understanding basic newborn care, trouble shooting breastfeeding problems, and being able to do light housework. While I would like to increase my lactation knowledge, I think I could certainly do all of these things.
I'd be curious to hear about what paths others have taken to become PP doulas.