I read this series in 1998 or something, so details are a little blurry. First book was about this apprentice summoning a demon, turned out to be his master's friend, but during the summoning, an assassin snuck in and murdered his master. The friend wasn't going to stick around, but then he found out he couldn't do magic anymore, so he had to train up the apprentice to get his magic back.
Other notable things: pet unicorn, there was a book about playing a champion sport to not get killed, fire breathing lizard friend. And there was a whole thing about becoming a court magician because they thought it was an easy job, but it turned out there was a major army coming to invade the kingdom.