Regarding the cost, just a quick note, when you buy the hard copy directly from No Starch Press you get perpetual access to the ebook as well. That's maybe cooler than it sounds: at any given time you've got access to the newest (or close to it) printing of the book via ebook, and it's nice to get the little updates and errata that accumulate over the years in a big book like for example, The Linux Programming Interface.

(at least that's the way they used to do business - I'm pretty sure they still do)