Covenants, p. 95, top of first column.
It is equally likely that I my understanding is off, so I'll try to explain my understanding:
The Cow isn't really any specific physical book - it is what is in the book, the contents.
Each Calf is a physical copy.
Basically, whomever hold the 'Cow' (the rights to the book), decides who can copy the book, and what they can do with their (physical) copies.
If you have written a book and I have been given (or bought or awarded or whatever) a copy (a Calf) under the oath of the Cow and Calf, you still own the Cow. I can't (legally) make copies without your permission - but nor can I give my original copy away for free! I may use it freely, but I may not transfer it without your permission. Much like a lot of modern software EULAs.
Most books are bought by Covenants, meaning every member of the covenant will have acces.
If I (as a single magus) buy a book under the Cow and Calf, I doubt anyone would complain if I allowed other members of my covenant to read the book, but technically that could've been a part of the specific Cow and Calf agreement.
So what does it matter if the Author or his covenant holds the Cow?
As long as everyone get's along, not much really. After that, a lot.
If I have donated a book to the covenant library, along with the Cow, the covenant owns the rights to that book.
If I leave, the covenant still owns the right to that book. They can decide to copy it and sell the copies for vis - or give them away for political gain. I cannot trivially offer the next covenant i wish to join a copy of my work, because I do not own it.
If I enter a feud with another member of the covenant, he still has the right to make copies of the book I wrote, and sell them or give them away (maybe, depending on charter)
If I donate a book to the covenant, but retain the Cow, the situation is different.
If the covenant wants to copy that book, for sale or for gifts, I have to give my permission - the charter may well hold sanctions if I decide against it, but they matter little if I have eg. left the covenant.
If I enter a feud with another member of the covenant and he copies my book and gives these copies away to squander what political capital I was hoping to gain by using it as a gift, he's broken the Cow and Calf, and whatever enforcement it has should move against him.
If I decide to leave the covenant and buy my way into another covenant by offering them copies of my books, I can freely do this, as they are mine to give away.
And that ignores entirely the issues of inheiritance if either the original author passes away or the covenant falls.