Baen publishing has had a long standing policy of releasing the first book or two of each of their series. They also include CD's in many of the hardcover releases that include all previous books of the partcular series.
Formats include Microsoft Reader which works on PC's and Pocket PC's, HTML which work on any device that includes a browser, Rocket Ebook format, Rich Text Format and still another for Palm handhelds.
This service has been around for years so many of you reading this will be well aware of it but for those that do not know if it I pass it on again.
http://www.baen.com/library/
I would currently guess that I have over 150-200 novels in E-format currently and while I still love a good hardcover it is so nice to carry an entire library around in my pocket.