Obviously the tech world is worse than most for how rapidly information goes stale, but still -- compare working with a few copies of things like Netscape 3 For Dummies, Learn Windows 98 in 21 Days and the like with what you can find in 10 minutes searching online.
Also, most books (regardless of the topic) are actually pretty poorly done. Even most instructional books written by experts are bad; the skills in question don't often overlap with the difficult skills involved in effective teaching something complicated with only static text and graphics.
If you want a really good book, you choose your topic then start reading reviews & recommendations online before you buy something.
If you're in prison you won't even be able to effectively research what the good books are, never mind finding them.