John Scalzi, The Android's Dream, Tor, 2006.
This novel is not related to the author's Old Man's War series. It is a story about the misunderstandings that can arise in inter-species diplomacy.
Agreements with alien species can be strange. In this case, the aliens require a particular breed of sheep to be used in a ceremony. An agent is sent to find the sheep, and ends up protecting a woman with a bizarre and disturbing background, which she didn't know about, while politicians and factions among the humans and aliens struggle to control the fate of humanity.
This book manages to be both very funny and great fun, while also discussing culture clashes and the wide range of human deviancy. The characters are engaging, and the plot reaches a satisfying conclusion.