This is one of those horribly complicated books that is simultaneously strong and weak in the same exact areas at the same time. *groan* 
 I mean, it starts off strongly with fantasy-type trials and tribulations in the empire, a king dying and his son being supplanted by the king's best friend, taking over the kingdom. Pretty standard... but then the whole other part of this novel is chock-full of purely wonderful heavy SF ideas that isn't entirely obvious at first but then becomes an infodump masterpiece of oddness and wonder and a world that really belongs as a movie just because the visual elements are completely amazing. 
 The world. Oh my lord, the world. Layers and layers and layers with ancient species and high tech and even ascended species. These humans are only on some outer layer. The infodumping doesn't do it justice. 
 Just... wow. Plus there's also different factions of the Culture, Special Circumstances against the rest, and no one seems to agree how to deal with people. :) And then there's the rogue Culture fragments that may or may not be in with the actual culture (either side), and the sister of the poor deposed kingling decided to quit Special Circumstances to help him out. 
 Everything else just devolves into a huge technothriller with huge stakes and ships and some really amazing descriptions and adventure. 
 So why am I only giving this a 4 star? 
 Because while the ideas are amazing and this author is known for his wonderful characters and his ability with traditional fiction, too, the character's names are too difficult and the ideas are too info-dumpy rather than a flowing masterpiece. 
 And to be entirely fair, I don't know how he could have done it better except by turning this novel into something much longer and gentle. 
 So it feels flawed and utterly brilliant at the same time. Which is a shame. I really want to LOVE this novel, not just appreciate it to death. Which I do. Hell, I want to kind of worship it, but I can't quite LOVE it. How frustrating. 
 I'll keep going! For straight ideas, Banks is one of those grand masters of SF. :) His characters, for their flaws, are still some of the most richly imagined. The plots are usually mind blowing. 
 And if he could ever keep it all flowing and working together right without tripping over each other, I might start worshipping the man as a god. :)