It makes me a little sad when people joke about Lucy's discovery of the "Book". It took me a minute to fully understand the reasoning behind its inclusion but I believe it serves as a way to show Lucy's start into her grooming process.
Usually, the first step groomers take when trying to entice minors is to show them illicit/explicit content with the intention of enacting them later on.
This is what we see in the later chapters of SBR. Where Valentine sexually assaults Lucy and does MANY heinous things towards her. While Lucy had been traumatized before this already (I'm counting the blackmore fight as traumatizing) I believe the "Book" serves as her first step into Valentine's sick assault process via Araki's way of "Show not tell".
I understand when first reading, it comes across as jarring and maybe a little comical with its abruptness but I think it's important to remember that this is a 14 year old girl's trauma we're talking about here. Even if it's all fictional, this kind of thing does happen to real people and I don't think we should take it too lightly.
