You may also want to consider buying a small mp3 player that also plays audio books from www.audible.com. You can get some for less than an ounce now, I belive. If you go to their website, they list compatible devices. You can also search their library, which I have found to be quite extensive. You can download individual books, or for $20/month you can downlaod 2 books a month. That is a great deal considering many of the larger books on tape normally sell for between $30-$80.
I know it's not quite as tactile and 'romantic' as a good paperback, but I have come to prefer audiobooks when hiking. Besides being able to listen to the book on the trail, I can listen while lying down or staring at nature--no more kinks in my neck from staring down at the pages, and I spend more time staring at and enjoying the beauty around me.
I personally use an Audiovox SMT-5600 Smartphone, available through Amazon for $100. It's about half the size of a Palm Treo 650, it's only 3.5oz with battery, and it plays audible.com books. Sometimes I take headphones, but often I just use the speakerphone. I average 17 hours of use on a single charge with the phone portion turned off (much better than any color Palm or Pocket PC phone I've ever tested).
I also use it with a bluetooth reciever (Globalsat SiRFIII, also with about 17 hour battery life), and download 7.5' USGS quads using OutdoorNavigator.
Of course, you can also read eBooks too, but you use more battery with the backlight on than you do with audiobooks.
I also download podcasts, such as from www.trailcast.org, and sometimes mp3s (though I much prefer listening to audiobooks than music). Heck, I sometimes even transfer a movie onto my phone, though that's usually for the kids on a long car trip.
You can also purchase a bluetooth keyboard for this, but I have not tested this, and I imagine it has the same problems as listed above with not being very steady on one's lap, but I like knowing I could do office work if I really had to.
I know this is beginning to sound like an ad for this phone, but I have gone though quite a few phones and GPS's, and this is by far the most elegant solution I've found; small, lightweight, great battery usage, rugged, a very stable OS, and as full-featured as a PocketPC phone (minus a touch-screen and qwert keyboard, but that makes it more rugged IMO).