I believe the current motivation is to establish a reciprocal Library Partnership with Kislev like we made with the Mootland Genealogical Library, so that both parties have access to each other's full catalogue into the future. We already had an immediate example of BOOK boons paying off with the Elementalist lore opening up a whole new perspective, maybe that didn't trigger your interest but there was plenty others that experienced catharsis from that. The boon would save us the most precious resource (AP) so we don't have to make book sharing agreements separately.
I really don't think Partnership automatically means having
full access to each others' catalogs. Because if it did, it'd mean the Mootland Genealogical Library would have access to all our books on magic (dark magics included).
Boney's said before that the amount of books we get access to from any given library depends on how well negotiations go (which themselves depend on how impressive our library is comparatively, whether we have things that are useful to them, whether they're just running out of space and want to make some room for more stuff, etc), and that means that in a
regular negotiation, neither party will have full access to the full libraries.
The Mootland Genealogical Library is, however, relatively small and K8P already has a sizable Halfling population, so it's not a surprise that they acceded to let us access what is presumably most of their catalog - that's kinda why they were the first ones we approached. We also didn't engage in a reasonable negotiation with Nuln - we had Boris do a sprinting jump-punch to the faces of each of the libraries, so we have full access to
everything they have and they have no access to anything that isn't one of their own copies.
Similarly, here, we are the ones dictating the terms. We can definitely make it a Partnership, but even in the most favorable terms to Kislev I don't think they'll have access to
everything KAU has to offer - Mathilde is obligated to not spread Collegiate secrets.