Well, for one, they were held accountable even though they were a victim.I mean, by the way people are talking here, why aren't the the ones that brought a dangerous weapon and sold it to Candlekeep at all culpable?
If I buy a loaded gun and then sell that loaded gun to someone else, I'm culpable as well, no? After all, I didn't bother checking that it was real before I sold it. Yet, there is no sense whatsoever that those that brought the books into Candlekeep are being held to any sort of responsiblity other than the fact that the book they used to gain entrance isn't there anymore.
This is where reasonableness comes in.
If I buy a gun and give /sell it to someone, the gun goes off and shoots someone (on its own) because I didn't check it? There's a chance I might be held liable (depending on applicable law) because a reasonable person would assume a gun is dangerous.
But a book? If I sell it to someone and they use it as a club to bludgeon someone to death, am I, the seller, liable? Likely not, no reasonable person would think the book is dangerous as a bludgeon - at least more dangerous than a typical heavy item.
Now this is D&D though. So let's say the book has explosive runes which then killed /injured someone. Then liability? Maybe. Or poisoned ink, likely no liability, but maybe?
This could get a bit in the weeds.
The actual adventure? The group must decide on a course of action and go with it, then roll with whatever happens.
Frankly, I like the multiple outcomes that could result from this scenario.