Thanks for all the responses.

I've just received the book from amazon today
"Celestial Navigation When Your GPS Fails"
by Mark Breach.

Even though amazon says 144 pages, it has
a lot of charts so the real reading is only about
70 pages or less.

Now I'm thinking about getting a sextant like
the one at Landfall Navigation ($127)
http://www.landfallnav.com/-nd026.html

But then I think another back-up GPS receiver
won't cost that much more :-)