Technical books in print are generally better formatted than their e-book cousins (O'Reilly's digital library, Kindle) unless they're PDFs. This has a major impact on readability if there are significant amounts of diagrams or code samples or tables in the text.
When something isn't just flowing text, it can be a real crap shoot in ebook form. And, even if the formatting is fine, I often find technical or other reference books (e.g. cookbooks) generally work better in physical form.
