The nicest surprise for me was to discover that for Amazon Prime members, Ryk Brown's Frontiers Saga is in the lending library. For those that don't know, Prime members with a Kindle (the real one, not the Kindle app on a smartphone, etc), can read a free book a month from the lending library list. How do you know which titles are on the list? Ah, Amazon buries that deeply on their site, making it not very clear. But suffice it to say, I've enjoyed almost all of the first 15 books so far for free, at the leisurely pace of one a month, interspersed with other interesting titles I've found. I'm just about to finish "After On", by Rob Reid, which was mentioned recently on TWIT, and I just purchased the 3rd volume of the Salvation trilogy, so I have plenty of reading material on hand.
Past free books I've read include all seven volumes of the Harry Potter series.