More and more people are getting Amazon's Kindle Reader and it is pretty simple, easy and inexpensive to get your book into this digital format. Kindle 1 blew off the shelves.
Kindle 2 is even better. Mobipocket is a digital format that can be read on Blackberry and a number of other PDAs, as well as on desktops and laptops.
Amazon also owns the Mobipocket system as well as Kindle, and offers free software to convert your book file into both the mobi and Kindle format (they are really both the same format, just different distribution systems). The free Mobibook Creator software is easy to use and can convert your Microsoft Word, RTF (rich text format) or html file into the Mobi/Kindle format.
Here is a good resource to help you get into this digital publishing revolution: