I really liked the Black Magician trilogy. I'd love to read more books that have 'evil' magic as a major component of the plot, preferably with some kind of trope subversion.
DragonLance chronicles trilogy, followed by Dragonlance Legends trilogy. It features a superb character arc with a dark magic user that arises out of an amalgamation of other wondrous character arcs, much like the DnD campaign of the same name.
Not sure how easy it is to get old comic books now that digital is an option but there was a whole doctor strange arc where he sorta got tainted and had to increasingly rely on dark magic. To me it was on the level of iron mans armor wars and I really wish both had been used with the movie arcs.
The Laundry Files by Charles Stross are rather good. It's a Cthulhu Mythos like world, seen through the lens of espionage, bureaucracy, and computer science. The magic starts off fairly low key, but as the books progress it becomes a bigger and bigger component of the characters. Human sacrifice and the maddening effects of magic are fairly consistent themes.
The ending leads over to the second trilogy, which IMHO is better and more mature than the first, it plays a few decades later. I wasn't exactly a fan of how the first one ended (though it was rather predictable) but it really sets the scene.
The prequel is good, and tbh if you consider reading the second trilogy, I'd read it as a filler in between, as it's explaining some of the dynamics with neighboring kingdoms that become important.
Oh I tried another series by the same author - the age of the five - and found it extremely dull. I started the first book twice and never got more than halfway through, then gave up for good.