@sergio, as Dwanthy mentioned, my work with <pre> tags had to do with recreational novels, not with books full of code examples. Books of code need to retain the formatting of the code within the <pre> tags. If it doesn't look good on your reader it means the author of the book was expecting you to use it on a larger screen.
You could try playing with the css for <pre> and make the font size smaller for the <pre> tags so that it all fits on the screen. Maybe use your reader in landscape mode as well, if it has that feature.