Hi Richard,
Shirebooks used to do a good introductory book called 'Hillforts of England and Wales' by James Dyer (ISBN 0-7478-0180-0) and it was only a fiver, but I can't see it on their website anymore.
http://www.shirebooks.co.uk
You might still be able to pick up a copy somewhere though.
BTW - Is that paraglider a 2 seater? ;-)
-Chris