I know everyone's decided that asking for books as payment is a bit gauche, but...
I think there's a compelling argument to be made that Kislev in particular could really stand to benefit from KAU's mandate to copy all of their books. This is a country that periodically faces down Everchosen and gets its cities razed, such that they have to build up again each time - wouldn't it be better if they could start that rebuilding with copies of all their lost libraries available at the safest library in the world?
I'm not saying that this is a better pick than mandating male magic users get sent to the Empire without exception, but if the deal explicitly included the right of Kislev to get copies of those books back whenever they're lost, I think it'd honestly really help them out. I can't think of a better example of KAU's explicit goal of preserving libraries through disasters.
We could make a deal where Mathilde says she's going to come around in 2 years or so to make that case, and that Boris will be so "convinced" of this argument with the threat of an Everchosen approaching he'll agree to all of the libraries, ever.
Yes, I want all of the Kislev books - but Praag was lost entirely and all of its works with it. The Fire Spire's books are only potentially not lost by an incredible coincidence. What if Praag falls again? What if the Everchosen makes it further south next time? There's no northern dwarves to harry them from the rear, now. Kislev stands to lose so much, loses so much every time, and this would help with that.