The Chronicles of Old Guy by Timothy Gawne
Price: $2.99 USD. 77180 words.
Published by Ballacourage Books on December 10, 2012. .
In the distant future mankind creates sentient cybertanks patterned on the human brain to help fight their alien enemies. Then, inexplicably, the humans vanished. They just went away. All that is left of the human empire are the cybertanks who, in their own way, keep the human civilization alive.