I had an inquiry regarding resources about Ad Orientem posture in relation to the General Instructions of the Roman Missal (GIRM). I've written about Ad Orientem a couple of times before, but here are some additional links.
First, go HERE to Fr. Zuhlsdorf's Blog---He has answered this question many times. When you go to this page, be sure to click on the link half-way down that says "What does GIRM 299 really say?" Or for more information go HERE.
Next got to this LINK on Amazon that allows you to read Chapter 3, "The Altar and Direction of Liturgical Prayer," of Cardninal Ratzinger's "The Spirit of the Liturgy."
HERE is a link to my blog where I made a very big post about Ad Orientem. It talks about all aspects of it, and I quote the Holy Father and his writtings about the subject (Spirit of the Liturgy), along with others.
For any more reading on this topic please read:
“Turning Towards the Lord” by Fr. Uwe Lang
“Spirit of the Liturgy” by Cardinal Joseph Ratzinger
(I quote these two books throughout my blog entry)
There's a case study regarding canon law, in which the bishop of Birmingham, Alabama decreed that Ad Orientem was NOT allowed, especially at televised Masses (remember this is where EWTN films their liturgies). Follow THIS link to see how canon law 838 pertains.
HERE is Fr. Longnecker's Blog where he has posted his letters to his parish about implementing Ad Orientem at Mass. Fr. Longnecker is an excellent resource as well.