A Working Library says something very true very eloquently:

On the page, the rhythm of the text emerges from both the macro design—the pleasing shape of the page, the proper amount of thumb space—and the micro—the right amount of leading, the evenness of the word spacing, the correct break of a line. On the screen, the rhythm of a text encompasses all of these things and more—the placement of a link, the shift from text to video and back again, the movement from one text to another. The rhythm becomes more complex as the orchestra gets larger, but the desire for rhythm does not subside.
In order to create this rhythm, the book must be designed and composed for the screen. A beautiful digital text can no more be arrived at by “converting” from a print design than a beautiful print book can be created by converting a Word file. The digital book will never come into its own so long as it is treated as a byproduct, unworthy of attention.
Furthermore, digital books should no more adhere to identical designs than their print counterparts; different types of writing, different voices and tempos, require unique approaches to design. The current crop of ebook formats were designed for the novel, and on that they do a fine job; but countless other texts—cookbooks, technical books, graphic novels, books on art, plays, verse—are rendered unreadable by that conformity. If the form of the book is changing, it ought to lead to more variety, not less.
Amen and amen.