As-is, I don't see much reason to give the Liber Mortis to Roswita beyond pure sentimental value.
She already has the practical benefits. Mathilde gave her some tutoring on fighting necromancers and the undead during their road trip, Vampires in Sylvania have a habit of suffering terrible accidents when Mathilde is around, and Mathilde has personally studied two Vampiric spells (fog form and remote-control) and suggested countermeasures which both wizards and non-wizards can make use of. The only thing Roswita barely knows about is the Skaven. But Stirland has barely any Skaven underneath it, for some reason.
(It would be kind of funny if that was once again all Manhavok's fault. God of Floods always hitting nails with the only hammer he has, fighting the good fight by flooding tunnels and basements, and everyone hating him for it.)
In order to get any more benefit out of the Liber Mortis than she indirectly has already, Roswita would actually need to hire someone with the capability to use magic. Someone incredibly trustworthy and loyal to her family, and proven to be nigh incorruptible by power or wealth. Hmm... can't think of anyone. Oh well.
I'm not entirely opposed to passing it on to her out of sentimental value at some point. But I can think of a better way to 'redeem' the thing that puts Mathilde's unique position to use.
Talismanic Runes
- Master Rune of Balance - Forged in the embers of a captured book of spells, this rune hungers after the Winds of Magic.
Assuming this thing scales off the strength of the book used (and in this case it's the freaking
Liber Mortis), we could go all-in with whatever Kragg Favour we've got and tell him we want this destroyed/transformed into the most powerful Runic anti-magic talisman he can possibly forge, as a gift and family heirloom that the Van Hal family can finally be truly proud of. Something to make every necromancer in Stirland regret the day they ever grabbed a fistful of Dhar.
I could imagine Roswita appreciating that, and it would make a heck of a way to turn the dark legacy of the book on its head. Even if the only explanation we ever could give for its existence is, "Your father wanted you to have this."