The Hundred Thousand Kingdoms and The Broken Kingdoms by N.K. Jemisin
I actually just finished the second book, The Broken Kingdoms. But reading it made me want to reread the first book, so I'm reading The Hundred Thousand Kingdoms right now.
I'm a huge fan of these books. The Hundred Thousand Kingdoms swept me completely off my feet - I was carried away by the story and the characters. At the risk of putting off any male readers, that book has everything a girl could want in a story. Actually, it might be okay if the male readers are put off; I always felt like The Hundred Thousand Kingdoms was a distinctly feminine book. I don't mean in a trashy Twilight kind of way - the main character is a great example of a strong female lead - but the themes, the obstacles, and the inner action create the feeling of a story by women, about women, and for women. It's fantastic.
The Broken Kingdoms is only tangentially related, so The Hundred Thousand Kingdoms can stand on its own; you could stop there if you wanted. The second book explores the world and the system of magic in greater detail. Personally, I think the first book is superior, but I still enjoyed the second book, and I'm excited for the third book to appear.