The Lives of Tao

The Lives of Tao (2013) is the award-winning first book in the Tao science fiction series by Wesley Chu. This has a fascinating concept of aliens who crashed here early in our planet's evolutionary timeline and who have been trying to get off and go home ever since. They cannot endure our atmosphere and survive by living in host bodies. In this way they have directed human evolution and history. Just know in advance that this isn't a feminist-friendly world; everything -even the female characters- is from the male perspective.

from the back of the book:

When out-of-shape IT technician Roen wakes up and starts hearing voices in his head, he naturally assumes he's losing it.

He isn't.

As of last night, he has a passenger in his brain -an ancient alien life-form called Tao, whose race crash-landed on Earth before the first fish crawled out of the oceans. Over the millennia his people have trained human heroes to be great leaders, to advance our species at a rate far beyond what it would have achieved on its own. Split into two opposing factions -the peace-loving, but under-represented Prophus, and the savage, powerful Genjix- the aliens have been in a state of civil war for centuries. Both sides are searching for a way off-planet... and the Genjix will sacrifice the entire human race, if that's what it takes.

So now Roen must train to be a hero worthy of his unwanted companion. Like that's going to end up well...

The Huffington Post has a positive review. Strange Horizons concludes, "This is the science fiction equivalent of beach reading, and I imagine that it will find solid success with a broad swath of SF readership." Fantasy Book Review describes it as "an action packed, secret organization, globetrotting sci-fi thriller."

SF Signal gives it 4 out of 5 stars. Publishers Weekly says, "the execution doesn't live up to the concept."