Because after the author has spent a year putting words to the page, getting it printed, putting it on the shelves, getting positive reviews, and getting word of mouth out there, the software I'm using has gone from 2.x to 3.x, the config file format has changed, and it's compatible with the thing I couldn't figure out already.

I could understand buying RHEL books, since RHEL (by virtue of 'tested and supported' software), is typically a few years behind the times anyway, or 'starter' books for people who aren't familiar with the topic, but there are so many good howtos, config examples, setup scripts, utilities, IRC channels, etc. that spending $80 on a book that will likely only come in handy two or three times before it's completely out of date just makes no sense at all.

Software these days moves too fast for dead trees to keep up. Give me a Safari Bookshelf app on my iPad and maybe we can talk.