Serenity: The Shepherd's Tale presents the long-awaited origin of Shepherd Book. Throughout the TV show Firefly and the movie Serenity, Book was portrayed as a man of the cloth who seemed to be a little too familiar and comfortable with the seedier side of humanity. He tried to devoutly follow the word of God, but had no problem picking up a gun when the situation warranted. ("Preacher, don't the Bible have some pretty specific things to say about killin'?" "Quite specific. It is, however, somewhat fuzzier on the subject of kneecaps.")
The book, it seems to me, is both a great success and a great failure. It is a great success in that the story is well conceived and well executed. It provides an intriguing back-story to Book and adds quite a few layers that I don't think any fans would've expected. From a craft perspective, it's also quite interesting how well the story flows backwards hitting on key touchstones throughout Book's life, each glimpse providing a little new insight to the character. The characters readers would already be familiar with fall perfectly in line with how they were portrayed on the show.
The great failure, however, stems from the fact that it's entirely enmeshed in that same Firefly/Serenity continuity. From two of said touch points being direct references to the movie and show respectively to the ending only really having some impact based on the reader's familiarity with the character beyond this book. I think there's plenty there that would keep a new reader from being lost, but I think the only engagement one might get out of the story comes from knowing Shepherd Book already.
That may be calculated. They may have looked at the sales numbers on the previous Serenity comics and realized, "The only people who're buying these were fans of the show." It could be that no one picked up any of those previous comics unless they'd watched the DVDs a dozen times over, and picked up this book knowing exactly what it was about.
Then again, the show's fandom often quotes from the pilot episode: "We have done the impossible, and that makes us mighty." So maybe it's okay that Shepherd's Tale doesn't seem conventionally inviting. Maybe the strength of the characters and premise IS powerful enough to sway over new fans. Maybe there is no power in the 'verse that can stop it. Maybe you can't stop the signal.