Consider Phlebas is the first of Iain Banks' speculative fiction Culture novels. It is space opera on a grand scale, set in the middle of an immense war between two galactic empires, the Culture and the Idirans.
The plot centers around Horza, a humanoid shapechanging agent of the Iridans, who undertakes a clandestine mission to a forbidden planet in search of an intelligent, fugitive machine whose actions could alter the course of the conflict.
The book is very compelling, in large part because it is morally ambiguous. While Horza despises the machine intelligences and moral laziness of The Culture, his embrace of and alliance with the Iridans reveals them to be intolerant, racist, religious zealots.