Most of the books I have on my Kindle were free. I happen to love Classics, which are out of copyright!

There are also a large number of very cheap books - most of my purchased ones were 89p and on eBay you can buy cds with thousands of Kindle books on them. That's not really how I'd do it because the books that I would want to read on the train tend to be very specific rather than just something I picked up ... if you see what I mean. S