The book intertwines multiple story tracks following the Culture's Minds (Sentient computers of unimaginable power), human diplomats, alien races, conspiracies, competitions for supremacy, innocents, and warriors. This leads to the one downfall of this book - too many characters to keep track off. I was confused for a lot of the book but, in a way, this allows the reader to feel what the Minds feel. They too are confused as to the meaning of the excession. Even with their superior intellect they have difficulty following who is on whose side and who is good, who is bad.
The book ends with all the threads coming together, some in unexpected ways just as an outside context problem would, to a satisfying conclusion. The one thing I would change is the epilogue. The last page provides the reader with some explanation of what the excession really was. I think the excession should have remained a mystery, a mystery leaving you wanting for more just like the characters in the book.
Recommended.