The Android's Dream by John Scalzi

A human diplomat kills his alien counterpart. Earth is on the verge of war with a vastly superior alien race. A lone man races against time and a host of enemies to find the one object that can save our planet and our people from alien enslavement...

A sheep.

That's right, a sheep. And if you think that's the most surprising thing about this book, wait until you read Chapter One. Welcome to The Android's Dream.

For Harry Creek, it's quickly becoming a nightmare. All he wants is to do his uncomplicated mid-level diplomatic job with Earth's State Department. But his past training and skills get him tapped to save the planet--and to protect pet store owner Robin Baker, whose own past holds the key to the whereabouts of that lost sheep. Doing both will take him from lava-strewn battlefields to alien halls of power. All in a day's work. Maybe it's time for a raise.

Throw in two-timing freelance mercenaries, political lobbyists with megalomaniac tendencies, aliens on a religious quest, and an artificial intelligence with unusual backstory, and you've got more than just your usual science fiction adventure story. You've got The Android's Dream.