V.E. Schwab, the shades of magic series is exactly up your alley, sounds like.

there are multiple realms that only a few can cross over - different versions of earth, with only a few connection points that overlap.

at least one realm, Black London, has been locked away because of bad things happening in history. those become clear as the series moves on.

there's another book that is kinda old, but involved some guy traveling through and ascending through multiple layers of reality, and his ultimate confrontation with the magic source.. maybe Imajica by Clive Barker, but I'm not sure that's the one (but very similar premise).

Raymond E Feist, Trudi Canavan are already mentioned.

But also: K.E. Mills - Rogue agent series starting with The Accidental Sorcerer

wheel of time and The Belgariad for sure. If you like nations quasi from historical empires then that will be a bonus with The Belgariad.

Robert Jackson Bennett is great at this. I'd say check out his Foundryside series.

Skulduggery Pleasant by Derek Landy. Been a fan of it since I was a kid, started rereading them from the beginning a few months ago and they're still peak. Great characters, great worldbuilding, great sense of humor.

i read The Black Magician series by Trudi Canavan back when they came out, and they may be a bit weird of a recommendation in that the magic there very much is "slam fireball into face" but there's a heavy focus on the process itself. Canavan writes mage battles like WWI gruesome trench warfare, with battle lines of wizards lobbing fireballs at each other for hours until their shields break, and supply lines running up to the front with mana supplies. it's an interesting take on the genre.

also, the Discworld books very much have magic as its own entity with its own will as a central theme.

Maybe Jonathan Strange & Mr Norrel? Or any of the Earthsea books by Ursula K. Le Guin.

Yay, one of my favorite genres.

In terms of the most bubble gum of it all (even if he actually does touch on some really awesome topics and concepts): Brandon Sanderson. We all lucked out that that man didn't get a job at Wizards of the Coast because he has made a career out of writing like twelve TTRPG fluff books per year and they are mostly all great. Basically all of his books heavily emphasize the Systems (magic) but the Cosmere books go above and beyond since the source of "magic" is kind of the overall meta plot.

And if Sanderson is bubble gum then Harry Connolly is shards of broken glass covered in fentanyl being handed out by a guy in a van outside an elementary school. His most "famous" series is Twenty Palaces that... was so hard for normies to get into that it kind of sent him into a depression and killed the series (there is one chapter in the third-ish book that is one entire run on sentence and it is AMAZING. But I fully agree with every single editor and beta reader that it was a bad idea. And, legend goes, it is what made his publisher and agent drop him). In that universe, spells are almost sentient creatures and their power is directly proportional to how many people know them. And while they are horrifying, they don't have shit on the creatures from the void which are just referred to as "predators" for every reason imaginable.

As a very early bits (will spoiler tag but it is like the first ten pages of the first book):

The good news is that Harry did a kickstarter a few years back and is going to finish off the series... eventually. He also has The Great Way which is "What if generic low magic fantasy but zombies. And then magic" which also very much depicts magic as an Other that corrupts its users. Nowhere near as good but probably a lot more enjoyable.

Now that we are all depressed and horrified: Stephen Aryan's Age of Darkness/Age of Dread are technically more about gods than magic but tend to focus on magic using viewpoint characters. Most of the books are either the result of a potentially extinction level event arising from magic or the expected backlash when the people find out about that. And... going farther into that gets into spoiler territories. I am not sure if it is exactly what you want, but it is close.

And as an honorable mention: The first 2-4 books of Steve McHugh's Hellequin series is almost exactly this. Nate Garrett is a super powerful sorceror. And if he uses his magic too much, it will be corrupted and he'll turn into a monster. And many monsters (like gargoyles) are a result of exactly that. And, as his magic power increases with every book, he gets closer and closer to the point that his magic gains a personality and it is a scary one. That said, McHugh quickly realized he can't write Goku if going Super Saiyan ends the series so he pivoted pretty hard (and then pivoted back a bit later...). I think it is still an awesome series (at the very least: It is the kind of series where the protagonist grabbing a glock and taking out a death squad of klan members is a massive hype moment and hope spot) but I really do wonder what would have happened if the original premise had continued.

And for another honorable mention with an even bigger asterisk: Mike Carey's Felix Castor books. For those not aware, Carey is basically THE go to guy for writing stories from a ridiculously Other perspective. His Luciter and Sandman runs in comics are endlessly praised and The Girl With All The Gifts became a movie that existed. In between that, he wrote some urban fantasy about an exorcist named Felix Castor. Magic itself tends to (maybe) not be corrupted and is more of an incredibly well defined, if intentionally nebulously sourced, plot tool for the mysteries that Felix is solving. But, light spoilers, it is intrinsically tied to the ghosts and demons (yup) that have arisen over the past decade or two. The books are more focused on human crimes (uhm... lots of content warnings there, by the way) with the demons and devils being an overall meta plot that comes to a head at the end of the initial run. That said, in 2023 he released a novella continuing the series that very much makes it clear that the origins of magic are being investigated and... it is almost definitely going to meet your criteria. So they don't count NOW but in a few years it will almost guaranteed be a perfect fit.

Check out Raymond E. Feist's books called Magician: Apprentice and Magician:Master. It introduces the character of Pug who reappears throughout dozens of other books by Feist.

Earthsea is what you want.

Off to Be the Wizard!

Magic isn't real per se, but a few techies figure out how to manipulate reality and pretend to be wizards using lights and special effects to make warping reality SEEM like typical magic.

Maybe mother of learning could fit. Atleast it has more interesting magic than just fireballs xD

Didn't get that deep into malazan yet but its magic system with warrens also seemed interesting