Coming back to Ars Magica since the 4th edition, I thought that you needed "profession: scribe" to be able to write. But browsing through the rules, I found that you need Scribe only to copy a text, while you apparently don't need it to write an original book, like a lab text, a summa or a tractatus. The only requirement being Speak [Language] at 5. Is that correct? And if it's correct, why do you need Scribe to copy a text and you don't need it when writing an original?
Profession scribe is needed to make sure you copy exactly what is there and don't add your own embelishments. Copying accurately is very much a different skill from composing an original piece of work even in the real world.
Ah, ok. I see now that Artes Liberales gives the ability to read and write. I seem to remember from the 4th edition that "read" and "write" were two distinct skills.
If memory serves, in the Covenants rules for writing, a Scribe with 6+ in the Ability is required for one point of a text's quality. So an unskilled magus writing a book on his own is going to make a slightly lower quality book, but it's assumed that in fact he's dictating to a scribe, or making notes on a tablet for the scribe to copy, or whatever. I believe a skilled illuminator and bookbinder are required as well.