It says that magicians spend time with their mentor to pick up new spells after levelling up, but if you have a mentor at character creation it will be the person that trained you in your specialty skill which comes from this list:
I’m looking at Page 10 and Page 99, second printing.
The mentor is (in theory) a fully statted character. (p10) The mentor can train you “in your specialty or a skill of the GM’s choosing.”
What I take from this is that a Magician’s mentor is probably a magician (or an elf, I guess). She has her own, non-arcanist specialty, but can still teach arcanist at the GM’s discretion.
This skill/mentor rule is cool for wizards. It meant that I had to have a cartographic Mage mentor in my first TB game. It was a good chance to wheel out some Borges themes - enough is always a plus for me!
We also had the Royal Mage as the Manipulative Mage Mentor/Actually Kind of Terrible Ensorceller in another game too.
Explaining why the arcane mentor is a renowned expert in some other skill encouraged us to consider the kinds of specialisation mages might have and some other aspects of being a magical professional in our settings.