I grew up reading fantasy and science fiction, and one of the greatest regrets I have is that I never found more than three books written by Lyndon Hardy, an author I still consider an automatic buy. In a way, this man's work defined what I expect from fantasy.
Of his three published works, I liked 'Master of the Five Magics' the best, followed closely with 'Secret of the Sixth Magic'. 'Riddle of the Seven Realms' comes in third, but keep in mind it's been over 20 years since I purchased the books, and I've moved six times and had to parse my book collection. I still have these wonderful novels, which I've re-read about once every five years.
Now about the book -- it follows Alodar, an apprentice Thaumaturgist. The magic system used by Lyndon Hardy is unique and self-consistent, hitting the right balance of rules vs power to make fantasy magic truly interesting. The closest modern equivlents I can think of are David Farland's Runelords or even Robert Jordan's One Power.
Alodar's journey takes him into enclaves of each magic practitioner across the land, and he meets adversaries that constantly beat him down. Yet he doesn't give up! I could really feel for Alodar by the end of the novel, so the primary characterization is wonderful.
The plotting was strong, and kept me with the book until the end. I never expected some of the twists thrown at me by Hardy.
As for the settings, I can still conjure up the inner heart of the Volcano with uncut gems waiting like burning fruit, the wizard's library with the spell barrier gongs, and the imposing black tower surrounded by minor demons. Those images have stayed with me for years, and have become benchmarks I measure new fantasy by. Lyndon Hardy hit the right mix of dialogue, pacing, and description I find lacking in so many 'modern' fantasy efforts.
If you haven't read this wonderful novel, I urge you to try it.