Reference books often don't transfer well to the small screen, no matter how well-designed they are. There's just not room to show before-and-after pics, or exploded diagrams, or with-and-without code samples side by side. And anything that involves a lot of flipping back and forth between pages isn't going to be great on an ereader; even screen scrolling isn't as good as page flipping for checking between two specific paragraphs.
Formatted poetry also doesn't translate well to the small screen. Aside from the hassle of all that custom CSS code to get the indents and spacing right, if the original poem was designed with a certain page size in mind, the small screen can lose a lot of the impact. (I've tried reading e.e. cummings on a small screen. Some poems translate; some just don't.)
Some art books are just never going to make the transition--I have a copy of Codex Serephinianus
, and I don't think it'll ever be a good ebook.
of Leaves is the only novel I can name off the top of my head that really doesn't work as an ebook. (Except, y'know, non-reflowable PDF, which really doesn't count.)