I took out a series of books in the late 90's that I believe were done by TOR. I am trying to track down the title of the series.
The story is essentially about mages who derive their power from crystals on their staffs and channel it in partnership with an owl familiar. When a mage died they basically turned into a sort of ghost that could still be talked to but was otherwise harmless until one of the ghost mages figured out how to possess one of the trainee ones. Can't remember why/how.
I vaguely remember there being a cross over of cultures where some mages discover a city that is essentially a giant tower. The books talked about all the pollution in the air above the tower and the people who lived there had highly advanced technology that the mages had never seen before.