Master of the Five Magics by Lyndon Hardy is just £0.78 at Amazon UK at the moment:
https://www.amazon.co.uk/dp/B01L5VB31C
I've read this a couple of times in paperback, and I'm delighted to have an ebook copy. While it is the first in a series, the series are all stand-alone novels. As I remember it, this is a complete story, and you aren't left hanging half-way through a plot.
If you like magic systems, I recommend this book. First published in 1980.