[davidw]

The problem with PDFs is that they make a lot of assumptions about layout and formatting, whereas mobi and epub are HTML based, and therefore work on a much larger array of devices - from mobile phones to Kindle for PC, including, critically, eInk based readers which are way better for sitting down to read than anything LCD based.

[nathanbarry]

Though if you are writing a book about design you often want the level of formatting and design ability that a PDF gives you. It's hard to give that up with an ePub.

[davidw]

If you need something to be pixel-precise, you'd probably just include a .png file.

eBooks aren't perfect for everything, though. I think a design book, for instance, is something I'd rather read as a paper book.