Little confused about the ability to learn from Ghosts in the 5th ed.
Under the Mysteries, a living ghost could seeming be a "living" respository of knowledge and teach and even train and apprentice.
In HOH:MC, in the Cave of Twisting Shadows, ghosts are all over the place (although they are really just a genii loci), could that spirit pretending to be a ghost act as a repoisitory of knowledge of all the grogs, companions and magi from the cave and impart this knowledge to others?
The Tremere in HOH:TL, don't want any bodies left out of Coeris because there spirites/ghosts could impart knowledge to others (and could be ill treated). Does that means a necromancer or other spirit master could coerce or otherwise persuade a ghost to impart its info through teaching?
Also in Mysteries, in Disciples of the Worm, they mention that secrets are wrested from spirits and ghosts of the dead. Could such ghosts teach abilities, forms, techniques, ancient magics...?
But, in Covenant of Calebis, seems to imply that ghosts can't really be good teachers because they are too separated from life and magi would need to start over at each new day.
So, do we have canon on what a ghost can do as far as teaching its abilities, powers and spells go? Could a merry band of nercomancers (or magi with good ReMe) get ghosts to train them in Creo (for example)?
This is my take on it, and others may (read: will) differ.
There are ghosts (Calebais and the kind that the Tremere worry about), and then there are ghosts (TMRE Living Ghosts and Criamon Twisting Shadow "ghosts"). The former are true ghosts, while the latter aren't.
True ghosts, for the reasons given on TMRE pg 72, cannot learn and tend to be distractable. Thus, learning something from a true ghost that would take more than an afternoon to explain (i.e. "teach me about Creo") is probably futile. You go home to sleep, and when you come back the next day, the ghost has forgotten who you are, forgotten that he's dead, and wonders why you are poking around his haunt. However, while learning an Art would probably be more trouble than it's worth, there are a lot of other interesting things you could learn from a ghost before its attention wandered (i.e. "What is the birthday and original name of the Exarch?" and "Tell me how to find the secret entrance to Coeris.")
As for ghosts in the Cave of Twisting Shadows, you noted that they aren't really ghosts; they're manifestations of a genius locus.
As for Living Ghosts, TMRE pg 70 contrasts them with "true ghosts" in that "the Living Ghost retains his consciousness, abilities, and the ability to learn..." I take it that whatever mystery allows the Living Ghost to retain its consciousness and will also allows it to pay attention to a particular subject long enough to be an effective teacher of Creo. (If it can be persuaded to do so, that is.)
Living ghosts are a special case: yes they can have apprentices.
The Primi are: the rest are ghosts.
Yes, it says in the chapter that they act in lieu of a library.
No, they are more worried about "Where did you hide the secret papers? What's the password for Coeris's first ring of defences?" That sort of thing.
This is the general case, yes.
Well, the problem you have here is that instead of treating each ghost as an individual with certain powers and abilities, you seem to be askng that all ghosts come out of the same biscut cutter. That's not how Ars Magica works. Even if I could say "All ghosts do X!" that would one be "well, so far..." Generally ghosts can't teach. Some, however, can. Ghostly warders, for example, IMO, can.
Note that this predates TMRE and its Living Ghosts which Timothy details above and also the Summoning/Commanding rules from RoP:tI. This latter source allows the use of Commanding twice a day for a season to force an entity (which could be a ghost IMO), to Teach the summoner one Ability it knows. YMMV.
I can't see the problem with commanding a ghost to teach you, although using the precedent of RoP:tI, it requires sufficient proficiency and time to be able to command a ghost for the whole season ie. 240 separate rolls in order to get the usual Teaching Source Quality, otherwise it's just Adventure Xp (RoP:tI, page 118).
I hope for your magus's sake that he can command a ghost without using stress dice as the statistical chance of even one significant botch is enough to give pause...