Standard Novels: This is perhaps the most "neutral" form of reading so we began here. Novels must be read in a linear fashion, beginning to end. There isn't any need to quickly flip back and forth, take notes, or memorize information. This leisurely engagement seemed to work well with the eBook reader. You can store a multitude of books so you can quickly flip between one another (if you're the type of person who likes to read more than one story at a time) and in general, it is easier to travel with the eBook reader than a stack of novels.
Instructional Books: Being a designer, we often use instructional books to guide us through new programs, languages, or any of the multitude of subjects instructional books could cover. We find that using eBooks has been very helpful in this regard. We can easily copy/paste script written in the books to implement directly into our own code, rather than carefully retype everything out. It is also very easy to mark pages or chapters in the book that we continue to return to time and time again for reference. And again, if we're traveling, it is much easier to pack up an eBook reader or laptop instead of hauling around the thick manuals.
Cooking Books: Similar to the instructional book, cooking books seem to benefit from a digital context. Some cook books are including links to videos to show complex instructions or, alternatively, because it is nearly free to include additional imagery in a digital book as opposed to a printed one, eBooks have seen much more helpful imagery compared to their printed brothers.
Illustrated or Art Books: Screens will likely never supersede a printed image in terms of color clarity. Novice or casual readers will likely not notice a difference but anyone with an acute eye will always recognize the beauty of a well printed image compared to that of an image filtered through screens and light and color calibrations and whatnot. This is why the art book market isn't very worried about the oncoming eBook craze.
Books for Research: If you're still in school or doing some serious reading which involves note taking and other forms of mark making (like highlighting) to relate back to the text, we've found it easiest to use printed books. We can quickly jot down notes in the margin and underline important information much more so than if we were to use an eBook reader. Technology is good for a lot of things but sometimes, it simply can't beat the speed of the hand.