I'd echo what ISM's said; reading from books is, for me, far easier on the eyes than reading from screen (and that's despite my having several laptops at my disposal, each with decent size/resolution screens and often configured to output to multiple displays).

I'd also be interested to know how much energy is consumed per consumer for an e-book to be produced, distributed, stored and displayed for reading, for the useful lifetime of that particular book. And that compared with the similar calculation for a paper copy. Given I don't buy books I'll only read once, I can't help but feel the traditional paper-based book may be, for the way I use it, rather greener. Of course, the point has already been made that technical books tend to be obsolete quite quickly, making the useful lifetime relatively short. However, I must admit that the technical books I tend to get are relatively "generic" ones (e.g. SQL language reference rather than T-SQL language reference as implemented in MS SQL Server v XXXX), meaning they're as useful as possible over the longest period possible.

Perhaps more relevant, though, is that if I have enough spare time available to sit down with a book (one of my luxuries in life, I'll admit), the last thing I'm likely to reach for is something work-related. Give me a good work of fiction from a skillful author any day.