To add to Darkwing's comment - Ars Magica uses the concept of the Mythic Pound (MP) - which is pretty much exactly what he says: 240 pennies to one pound of silver. It's a convenient monetary abstraction, and allows the game not to devolve into an exercise in medieval international banking.
You are correct: at its heart, a scriptorium is just a room with paper, ink, a writing desk, and good lighting. It's the minimum required for anyone with the Profession: Scribe ability to do their job without penalty.
HOWEVER - you can use the rules in Covenants to create a single-purpose Hermetic Lab which grants a skill bonus to the Scribe ability. This "lab" can, in fact, be your scriptorium - anyone with Magic Theory of...3(?) can do it (ie, they don't have to be Gifted). It takes a month or so to set up, and it's the medieval equivalent of Feng Shui: making sure the potted plant is in the corner, and the doors are aligned with the western sun, etc. Anyway, it adds anything from +1 to a +3 to the Scribe ability, which improves the Scribe's seasonal copying rate.
For the Roots (5/15 or 6/21), you could buy the entire set for a few silver - as mentioned above, "silver" is a pound of silver, and amounts to half a year's wage for most folks. (Fun fact: gold was so rare that very few governments bothered to use it for coinage during the middle ages.) If you just want to borrow a Root, you could probably do it for free - it's almost literally like asking a college graduate if you could borrow their freshman calculus book. The only reason these wouldn't be available to a magi is if there's something about the campaign you're in that prevents someone from reading them. (ie, you're off by yourself up in the outer reaches of Scandinavia, or something, where the Redcaps don't travel.)
For a book on the arts, you can use the Library at Duremar as a guide: while TECHNICALLY you can borrow anything from them for free (because they're a bunch of Bonisagus, and they're supposed to be all about the sharing), in reality you need to bribe the librarian with a pawn or 2 of vis. Beyond that, it's a roleplaying issue, and it depends on how popular the book in question is. Guardians of the Forest (the Rhine Tribunal) has more information on book-borrowing.
But to re-iterate: the Order is a scholastic organization - for the most part, they WANT people to read their books. The rules for Duremar are the extreme edge - you can actually borrow stuff from them for free, but you get put on the wait list. If you want it NOW, you need to bribe someone with the vis.
And also - the Branches (those 10/20, 15/15, or 20/10 summae), although useful, are relatively common - most established covenants (ie, anything but a Spring or early Summer covenant) will likely have a full set. And for the most part, magi will only be using a few of them at a time, if at all. And finally: magi aren't actually jerks: they're (boarderline Asberger's Syndrome) scholars who want to be acknowledged for their brilliance. As such, they're usually OK with loaning out the basic books (or else letting a travelling magi spend a season or two with them.)
In fact, "roaming around the Tribuanal for a few years, depending on the hospitality of various covenants" is the accepted norm for recently-gauntleted magi. It's the equivalent of a Gap Year for members of the Order, and most Covenants have spare labs for those folks. So walking into a Covenant and asking if you can stay there for a year or two and peruse their library is actually the NORMAL way of borrowing a book.
That being said, doing this does require that the travelling magi act in accordance to the Convent charter - and usually there's a clause in there about how much such folks have to contribute to the Covenant. And actually - yes, that might be a couple of pawns of vis. Usually, though, the default is "a season of service" each year, in which the magi has to spend some time doing some form of service for the folks who live there. That service might be "go out and harvest some vis for us" - so if you already have some vis available, you won't have to do that.
But usually it's something that doesn't actually take a full season - it's just something the other magi don't want to do - such as vis harvesting (which might take a week or two), or clearing out the redcaps from the local farie forest, etc. It's really an excuse for your magi to go on an adventure with some grogs, and get paid for it at the end.