Random House has added some books available for free download, including Red Mars, by Kim Stanley Robinson. While it's certainly not for everyone, I'm a big fan of this book. Be aware that it's the first in a trilogy, and that it does have some slow parts, a great deal of detail, and more than a little optimism. It also has some serious (if made up) science on what it might be like to actually visit Mars.
Anyone have comments on any of the other books there? I've enjoyed reading ebooks in the past (in my Newton and PalmV days).