For me, the readability of the on screen font is just fine. My big problem with reading a book is the usability of the reader. Most books are designed to be read as if printed on a page. That means that I spend a lot of time scrolling through a page, then I have to fish around for the "next page" button. Just like Pubb said, you spend too much time using the computer instead of reading the book. Some simple rules for me:
1. Use large type and put less on a page so that I can see everything on a page without scrolling (i.e. make the page horizontal, not vertical)
2. Allow the user to set two buttons on the computer for going foward and backwards. This will help replace the need for "thumbing" through the pages. That is probrably the number one thing that bugs me is not being able to easily thumb through the "pages"
of a book.
3. Yeap, a table PC would work wonders (but then we are back to making sure there is an easy way to thumb the pages. Sorry, I don't see using the pen to push a "button" on the screen as an easy way to do it. It would have to be physical buttons properly placed.)