Weeks is known for a previous trilogy (forget what its called) about assassins (possibly magical).
The Black Prism is the start of a new series, and its pretty good. It has some interesting concepts, a magical system that requires the user to be able to see specific colours in a prism and cast spells using a material called luxin. Spells are all physical, as you use the physical matieral luxin.
The most normal kind of "drafters" (as wielders of this magic are called) are only able to see and cast a one colour in the spectrum. Some special ones can see and cast two colours next to each other on the spectrum, and a very rare amount can see and cast multiple colours.
But only one person at any given time can see and cast ALL colours at once. He is known as the Lord Prism, the chosen of Orholam, the god of Weeks' world. A Prism is always found shortly after the previous one dies, and a Prism always lives either 7, 14, or 21 years after being annointed. A Prism is something of a priest-emperor, though more as a political figurehead than with any real power.
But all drafters, including the Lord Prism, have lifespans shortened according to how often they draft. The more and the more intensely you draft, the shorter your life span. The end of a drafters life is marked when he or she "breaks halo" (named because the coloured halo in the irises of their eyes breaks and the colour(s) they can draft seep into their eyes), and they turn into the dreaded and insane Colour Wights, people that turn themselves into creatures entirely of luxin, no longer human in appearance or temperment.
The story of the Black Prism is about the aftermath of a Prism War, where for the first time in known history there were TWO Prisms at once - two brothers. A war was fought between them. 15 years later, the seeds sown in that war are coming to fruition. A heretical rebellion is being stirred. The entire religious system with the Prism at its head is under assault. In the midst of this, the current Lord Prism is desperately trying to keep a secret that could threaten to create even more war and chaos within the religious and political system, as well as his personal life.
It's an interesting story, and I recommend it to any fantasy lover looking for a new good book/author to read. Weeks has a very witty style of writing, in a crude kind of way. As an example, in the beginning of the book a character is out at night and hears wolves. He describes it as beautiful and terrifying at the same time, but he says it as something like "It was the kind of beauty that makes you shit your pants"