Three years ago, I posted five of my ebooks on Google Books. I limited to 5 (out of 40+) in order to test Google Books -- primarily, how much money could I make off it.
The initial results were discouraging; the long-term results continue to be discouraging:
After three full years, I've earned $27.22 from 2,782 views. Google only pays once the total reaches $100, so I can expect the cheque in just 8 more years (at the current rate).