I'm pretty sure this has been addressed in at least one of the supplements, but just for clarification if nothing else:
You need to have a Language Score of 4 and an Artes Liberales Score (with literacy in the script in question) to be able to read a book in a given language and gain benefit (i.e. get xp from it); with a Language of 3 or less, you can still read it (as long as you have the Language and the Artes Liberales), but you don't understand it well enough to get the benefit. Most books in the Order are in Latin, but there are likely to be a few books floating around the Tribunal in Scots Gaelic.
[strike]You must have a raw Language Ability score of 5 to be able to write anything with a Source Quality (such as summae, tractatus, letters for the Correspondence rules somewhere). To simply write letters or whatever, you simply need the Artes Liberales and literacy in the appropriate script. Note, however, that with a Language of 1, it's going to wind up looking like the letter I just got from my six-year-old cousin.[/strike]
After some thought, reading on the forums, and looking over the published characters in the Main Rule Book as well as the supplements, I have come to the decision that using a Specialization in Latin (or another appropriate language) will allow the character to study from, or write, a book or other text. This stems, in part, from my observation that practically none of the published characters (including members of House Bonisagus) would be able to write books if this were not the case, and at least a couple are of an age and description that indicated that they had done so.