I've come across one called When Women Were Priests: Women's Leadership in the Early Church and the Scandal of Their Subordination in the Rise of Christianity (sorry I can't link on my phone) but not sure if it's what I'm looking for.
I couldn't find a googlebooks edition which although it only lets you see 50 pages often helps you work out whether it is worth buying. This review http://www.goodreads.com/review/show/279529657 might be helpful.
I had a quick look on Amazon and 'Ordained Women in the Early Church' by Madigan and Osiek looks interesting.
Ben Witherington has written 'Women in the Earliest Churches' If you google that with google books it looks like you can preview it online.