This is an excellent supplement for Birthright. To be honest, it’s a necessity as it explains the investiture process along with going into all the needed details of religion in Cerilia.
This book also does something amazing for AD&D 2e (and other editions) in that it goes through actual doctrinal differences within the churches dedicated to the same gods. It shows how different temples can be based around different teachings and yet still follow the same deity. Birthright has always been a fairly grounded setting (if not realistic) but this simple aspect really adds gravitas to the world.
Book really only loses a star because the artwork is sparse and the layout is rather poor. Priestcraft came out at the end of Birthright’s run so the money spent on the products definitely took a hit here.