The Liberal Case Against Abortion By Vasu Murti and Carol Crossed is an excellent place to start. http://www.amazon.com/The-Liberal-Case-Against-Abortion/dp/0977223434
It’s on amazon, but its really expensive. Fortunately, most of the book is available online here. http://www.all-creatures.org/murti/pub-thelib.html
I remember reading that some time ago.
You should also check out Feminists for Life
There are a number of other really good books on abortion written by people who are more on the left. One is http://www.fnsa.org/apaw/ Acheiving Peace in the Abortion War. http://www.amazon.com/Achieving-Peace-Abortion-Rachel-MacNair/dp/1440113254
You also might want to contact Democrats for Life, the Pro-life Alliance of Gays and Lesbians, Pro-Life Pagans (they are mostly liberal) and Secular Pro-life. These are all groups with members that are, for the most part, pretty far from conservativism
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