A recent issue of Wired clued me in to dailylit.com. dailylit.com has a bunch of public domain publications (mostly classical literature) which they have carved up into bite-size chunks deliverable via RSS feed. So you can go to dailylit.com, pick a book, and subscribe to it in your RSS reader. You get a post every day (or 3 times a week, or every weekday) which you can read in a couple of minutes. It's pretty cool. I'm reading Sir Arthur Conan Doyle's A Study in Scarlet (the first Sherlock Holmes story).
There's also the feature that each post comes with a link to tell dailylit.com to release the next post (rather than waiting until the next day). This feature has a caveat if you're using an online RSS reader like Google Reader: although dailylit.com immediately puts the next post in your feed, the post won't show up in your reader until the RSS service checks the feed again (which might take hours).
Be warned that the catalog at dailylit.com isn't nearly as extensive as something like Project Gutenberg. But the selection isn't bad, and it's a neat way to read a book.