Blood of Martyrs is the first installement of a range of back-ground books regarding the various "vocations" of inquisitorial acolytes (military, criminal, clerical (this one), law and psykers)
Luckily, Fantasy Flight Games picked one of the more interesting ones to publish first: Clerical. It begins with an overview of the history of the imperial creed, touching the important points in time like the Age of Aposthasy, Sebastian Thor and others. Also it presents more background options for clerical characters and alternate career ranks for everyone else, apart from psykers, obviously (yes, there IS an alternate rank for Tech Priests).
One of the most interesting chapters is the one about the Adepta Sororita. This presents some fundamental changes to the career path presented in Inquistors Handbook, as it introduces three "new" careers, the Sister Famulous, the Sister Dialogus and the Sister Hospitaler. In effect these are alternate career ranks for the adept (Hospitaler, Dialogus) or Cleric (Famulous) but with a twist. Also it presents more options for the Order Militant so that it is not too narrow.
The second change is to the faith talents as it is now an expanded Talent-Tree with one of the higher tiers requiring several of the lower tier talents. However as faith talents can now be obtained more often it fleshes out the uniqueness of people (not only Adepta Sororita but Guardsmen as well, if it happens...) touched by the Emperor's Grace.
All in all it is nothing which should be in everybody's library of the darkest of heresies, as most of the back-ground information can be obtained on the net, or one could come up with it oneself. However it is well presented (as usual) it gives GM's a nice foundation to base more of his faith based boons and challanges AND it gives the sisters of battle a more elaborate face than fanatical storm-crazed-flamer-wielding-heretic-stomping shrews.
Four Stars from me.