Not what you asked for (i.e., book recommendation) but if you can get him to open his computer and connect to the internet, then you can always use software like TeamViewer and control his computer and then teach him whatever he wants to know. Also gives a bit of a personal touch to the whole process
I did this with my mother : All she knew about computers was to pull the power chord if she felt I had slept without switching it off myself - she figured it was like switching off the TV

(RIP my movie downloads).
When I was away, I called her up and asked her to switch on the computer, connect to internet (thankfully modem etc was already set up), open up a browser, download TeamViewer and read out her computer ID and password (the one for TeamViewer). Once she did this, I went about teaching her how to do various basic tasks (browsing, checking email, video chat/skype, reading newspapers etc)
Took a few hour long phone calls, but now she can function on her own - the entire process and her current self sufficiency makes me feel so much better
