We've wondered for years how and when Miura might reveal the backstories of some of these big characters, including the major apostles, the God Hand, and of course Skull Knight. With the appearance of this novel, we might have our answer. And if so, that would be ... an unprecedented low point for the series. I think this is pretty evident, but just to be clear: Such stories should be revealed in the pages of the manga, not some new supplementary work being penned by a contract writer. Furthermore, there is no guarantee this work would be translated for international fans.
But like Aazealh said, we'll need more info to be sure. Right now all we have is the title, our overactive imaginations, and our sensibilities as fans having endured some poor adaptations in recent years. Best-case scenario, it's a fragment of Grunbeld's life after he became an apostle — filler material. Worst-case scenario, this book tells his life before he became an apostle, leading up to his sacrifice — in a book that's coming out in less than two months.
What worries me is that if Miura was trying to merchandise Berserk in small novels, there are a number of avenues he could take that wouldn't disrupt the flow of reveals, stepping on the toes of Berserk. What about Judeau's backstory? He's a fan favorite. People would devour that book happily, and it would be inconsequential to the ongoing series. Pippin's a popular character, too. And hey, maybe not the smartest decision, but I personally would buy a PALLET of novels about Corcas' life before the Falcons.
But why embark on Grunbeld (and really, he's one of a group, so there's an implication that there will be more of these) if not to tell the primary story of consequence about the character? And if that is indeed the plan, how would such a reveal be woven into the manga after the fact?
It's a very confusing move with big implications about the rest of the series. OR it's just merchandising that we will laugh about in a few months.