Because you're a kind soul and generous to a fault? 
But since you ask, I'll try to present as strong a reason as possible[i] (bald-facedly admitting that I'd really rather not lose either, but especially the Creo Summa)...
(And rather than "quote" anything out of context, I'm just providing links - these will be more familiar and significant to you than me.)[/i]
Okay, specifically looks like there are only two texts in question, only 36 "free" texts against 698 total Library Build Points...
- Principia Fabricata by Portia Verditii. Creo summa, level 9 quality 15. 0 bp
- Priscum Fundamentum, by Portia Verditii. Magic Theory Tractatus, Quality 12. 0 bp.
... those are the only ones from Portia, and the only ones listed with "0 bp".
I think it's important to note that these contributions (esp the summa) seem to have been taken into account "when the Library was being planned" - which is a critical phase of Covenant planning both IC and OOC. And a Creo summa is fairly central to any good library (and it already has some lesser holes* in it as it currently stands.)
(The Lib' currently is lacking any Perdo, Animal or Aquam summae.).
Most significantly, the words "donation" and "contribution" are used repeatedly, and I think that's backed up by Arya's statement that "...whatever purchases she's making are on behalf of the covenant rather than herself".
That seems correct on all counts.
I did a Search to try to find specifically where the books originally came from... and it looks like she spent a season (during CharGen). That was, apparently, in response to Arya's question w/ MT's answer below it here. (The change from the original 2 Tractati to a Creo Summa is explained by this, a later "fine tuning" revision Library-planning post, and was roundly applauded.)
Now, I fully admit that doesn't balance a player donating a pile of points and then disappearing, but these aren't "a pile" either, nor close. And it appears that, MT, you implied these were "donations" (your words) to the cause, and that sounds very IC rather than OOC - but I could be taking that out of context.
Looking at Portia as a character. She does have the -3 Flaw "Major Personality Flaw: Proud" - would that cause her to leave the lab notes as a symbol of her (however brief) involvement in the Covenant? Or possibly as a token of payment for her stay here, whether required or no? I don't see any Traits listed, so nothing there that would bear on it one way or the other.
So, myself, I'd have to conclude that Portia donated those texts to the Covenant permanently (rather than putting them there "on loan"), having brought them specifically for that purpose. (No mage writes a book for their own study.) And I also have to doubt if her "Pride" would let her take them back - tho' I am ignorant of the reasons (on- or of-camera) for her leaving.
Beyond that... I got nuthin'. 
Saga-wise, I have no great problem with it - maybe it was mice - altho' I've done the "And we need books" subplot a number of times, so it would be better if it were a subplot to something other than that alone.