There's a novel, or possibly a series, that I read years ago as a young teen. I've been trying to identify it for a while, but my search has been fruitless so far.
The novel is set in a fairly typical medieval setting with city states. Magic use is known, but is either outlawed within the main city state, or is only usable by the elite (who were also antagonists, if I remember correctly). The main character is a young man/teen who discovers somehow that he can use magic, and ends up having to go into exile because of it. He is taught more magic by an elf companion, and of course being the main character is just naturally good at picking it up.
I distinctly remember that there were different schools of magic in this system, with the one that stands out most to me being a combat magic where the user had time slow down for them, and their vision was altered to show where they should move next. There were also other uses of magic outside of combat, but it's been so long I'm having a hard time remembering many details. There was a scene in which the main character and his companion help to heal an injured unicorn.
I believe that before the main character had to go into exile, he had discovered a book of magic somehow, which was where he initially learned how to use it.
I know this is a pretty vague description, it's been around 15 years since I read the book, which at least narrows down the possible publication date to around or before the year 2000. I'd like to say it was probably written in the 80s or 90s, don't think it would have been any older than that.
I doubt it was the best fantasy around, but it's been a story that has recently been stuck in my mind and I wanted to see if I could find it for nostalgia's sake.