In last week's post about the books I've been reading on my iPhone, I neglected to point out that there are a ton of free books you can "buy" from Amazon. The Kindle app is a free app in the App Store, so go ahead and put it on your phone if you haven't already.
Now go visit this page (better, if you use iGoogle, My Yahoo, Google Reader or another feed reader, subscribe to the feed). This is the current best sellers page for the Kindle, and not surprisingly, books that are free tend to sell pretty well. When you see one that intrigues you, click to buy and it'll be on your iPhone in minutes.
I've got a backlog of at least a dozen books I've picked up over the last 6 months that all look pretty good, and several have been downright outstanding. It's a great marketing tactic for authors - particularly those who have several books available but who want to reach a new audience. And if you're on a tight budget you can't beat the price!
Update: And if you don't have an iPhone/iPod Touch, or you'd really rather have the actual Kindle, the original Kindle (refurbished) is now $149 from Amazon.