How much gold would it actually take to get all of said books? Because we've got a lot already, are going to start making a whole lot more, and I doubt it would take even a fraction of the money the ten wealthiest families In Marienburg have to offer.
Bretonnian Extensive The Empire (100gc), Bretonnian Extensive Trade (100gc), Dwarven Antiquarian Sevir (100gc, 2 DF), Imperial Antiquarian Divine Magic (100gc, 2 CF), Imperial Antiquarian Enchantment (100gc, 2 CF), Imperial Extensive+Esoteric Chemistry (250gc), Bretonnian Extensive Chemistry (100gc), Imperial Esoteric Engineering (150gc), Bretonnian Extensive Engineering (100gc), Dwarven Extensive Engineering (100gc), Imperial Esoteric Ulric (150gc), Bretonnian Extensive Ranald (100gc), Bretonnian Extensive Ulric (100gc), Bretonnian Extensive The Eonir of Laurelorn (100gc), Imperial Esoteric+Extensive The Ten Kingdoms of Ulthuan (250gc), Bretonnian Extensive The Ten Kindgoms of Ulthuan (250gc), Bretonnian Extensive Arthropods (100gc), Extensive Dwarven Aethyr (100gc), Imperial Esoteric+Extensive Agriculture (250gc), Bretonnian Extensive Agriculture (100gc), Imperial Esoteric+Extensive Naval Warfare (250gc), Bretonnian Extensive Naval Warfare (100gc), Dwarven Extensive Naval Warfare (100gc), Imperial Extensive Verena (100gc), Bretonnian Extensive Verena (100gc), Imperial Esoteric+Extensive Tactics (250gc), Bretonnian Extensive Tactics (100gc), Bretonnian Extensive Canines (100gc), Imperial Esoteric+Extensive Druchii (250gc), Bretonnian Extensive Druchii (100gc), Dwarven Extensive Druchii (100gc)
Total cost (including discount from budget): 3950gc, 4 CF
As for why this is all useful to Mathilde...
Agriculture - To help Panoramia with the Waaaghsoak mushrooms. (Bonus: Helps Panoramia with her personal agricultural efforts.)
Arthropods - Writing the book on the We. (Bonus: Helping Kazrik talk to one of Karak Eight Peaks' allies.)
Canines - Teaching Wolf stuff.
Chemistry - Helps with the Moulder chemicals and may help with the AV safety action through info on handling hazardous materials.
Divine Magic - Studying the Coin, plus follow up actions.
Druchii - For the elfcation.
Enchantment - For enchanting stuff.
Engineering - To study and write books on the plethora of our skaven artefacts. (Bonus: Helps Adela and Maximilian with their personal engineering efforts.)
Naval Warfare - To set up the paramilitary river navy.
Ranald - Studying the Coin, plus follow up actions.
Sevir - AV plus various other stuff.
Tactics - If we join the war in Sylvania it's not a small chance we'll wind up in command of an army. With her Martial and bonuses, Mathilde's probably better than all of Stirland's generals at fighting undead.
The Empire of Man - Lots of general use given the nation we're part of, but of greater relevance with the Marienburg situation.
The Eonir of Laurelorn - We've had to do diplomacy there and we may have to do so again.
The Ten Kingdoms of Ulthuan - For the elfcation and possibly (though IMO unlikely) relevant with Marienburg.
Trade - To deal with the peat problem and possibly of relevance with Marienburg. (Bonus: Helps Wilhelimna with the EIC.)
Ulric - We've had to do diplomacy there and we may have to do so again.
Verena - For the book-sharing action.
EDIT: Plus there's all the Estalian and Tilean books we can buy after we learn Classical.