Page "Ars Magica" Paragraph 53

Learning is mostly done using books to Study from.

Every Covenant will have a respectable library, since the magical tradition of the Order of Hermes is a bookish one.

One can also learn from Teaching or Training if you can find a suitably skilled individual, or one may Practice by one's self.

Magi who are able to write useful books or teach well can use these as commodities, trading with other magi for books or training.

Finally, a one may learn from Exposure while working a trade or performing lab activities.

This is the least efficient way to learn, but it is a nice side benefit to the result of the work done.