Ubuntu For Non-Geeks, 3rd Edition: A Big Thumbs Up
Computers are not intuitive. Computers are abstract, and trying to tie abstract concepts like directories, files, and interfaces to paper folders and files, and physical desktops creates more confusion. I prefer a direct approach: show me. Which "Ubuntu For Non-Geeks, 3rd Edition: A Pain-Free, Project-Based, Get-Things-Done Guidebook" does in a most excellent fashion.
The classic RTFM advice (read the fine manual) has never been good enough. It's better to point out which manual and which page or section. And then you are still at the mercy of the fine manual: does it really help the reader? Or is it a more typical geek-spew that almost, but not quite, makes sense only to someone who already knows everything about the subject?
Writing good howtos is hard. Computers are completely literal and don't fill in the blanks for you.