Reddit reviews The Books of Magic

Since you mentioned you're looking for books featuring DC's magical realms, Neil Gaiman's Sandman series (Books 1-10, Endless Nights & Overture specifically) is not to be missed.

They're easily among the best comic storytelling that exists in the medium, everyone owes it to themselves to read it!

EDIT: Just wanted to add a few more recommendations that came to mind, all of which are well worth anyone's time:

• The Contract with God Trilogy - Will Eisner
  
  
• Daytripper - Fabio Moon & Gabriel Ba
  
  
• The Books of Magic - Neil Gaiman
  
  
• Black Orchid - Neil Gaiman
  
  
• Planetary - Warren Ellis
  
  
• Doom Patrol - Grant Morrison
  

Check these out:

• The Books of Magic
  
• Lucifer
  
• Hellblazer
  
• Saga of the Swamp Thing

Gaiman also wrote The Books of Magic which has a similar tone as the Sandman series

Books not mentioned so far that I like:

Lev Grossman's Magicians Trilogy (not 1st-person, otherwise follows your criteria, Harry Potter-esque, some people dislike the protagonist but he's intentionally kind of a tool), starting with The Magicians.

R. Scott Bakker's Prince of Nothing trilogy (not 1st-person, magic is seriously powerful and the protagonist is an already-accomplished practitioner, mythology is complex, I don't recall it having a huge amount of characters), starting with The Darkness That Comes Before.

If you're willing to step outside of prose works, I like The Books of Magic a lot; it's a comic book miniseries.

The Sandman: Book of Dreams is a collection of prose short stories, edited by Neil Gaiman, and is pretty fun.

Destiny: A Chronicle of Deaths Foretold is a three issue mini series.

Sandman Midnight Theatre is a must.
And that might get you into Sandman Mystery Theatre, which ran for 70 issues.

Similarly, The Dreaming series, Lucifer, and The Dead Boy Detectives are all spin-offs.

And then there is The Books of Magic, also written by Gaiman if you want to stay in the same realm of DC/Vertigo magic.