I finished this one last. Faith and Fire follows a Sisters of Battle squad as they pick up a powerful pyrokene and bring him to his home world to be executed for his crimes. He had somehow escaped the notice of the Black Ships and gone on a murderous rampage throughout the subsector till he had been captured on Groombridge. Naturally he escapes custody and goes on another murderous rampage on his home world.
The plot is fairly standard, the interesting thing about the book is the look it affords us into the practices of the Battle Sisters, Cain describes the Inquisition as the Emperors pet psychopaths. If that is indeed true than the SoB's are his pet sociopaths. We see within the first few chapters that a Sister who fell prey to the pyrokenes psyker manipulations (she was mind fucked) was forced to sacrifice herself to save her squad from execution by volunteering to join the Sisters Repentia. A group of disgraced SoB's that go into battle in robes with chainswords, shepherded by a loony with a neural lash. And that's just one of the frankly wasteful and idiotic practices of the order.
The novel also provides some nice insight into what the Emperor was up to after he left the Great Crusade in Horus's hands and retire to Holy Terra. It's no secret that he intended for mankind to become a race of psykers, what's not clear is how he intended to do it. That is partly answered in this novel. A device had been discovered on Neva thousands of years prior that was simply referred to as The Engine, it's purpose: to turn normal humans into powerful psykers. The individual that winds up using it claims that he will become the second Sigillite.
I'd recommend the novel only if your interested in how the SoB operate and the insight into pyskers and the GEoM's plans because, unfortunately the characters are flat and boring. The SoB's come across as murderous loony's who exist only to spout dogma and fire meltaguns at heretics.