to link to cells across sheets or books you can simply:

type '=' in the local book
navigate to the cell(s) on the external book

it's a documented technique and works perfectly with previous versions of
XL - Excel XP seems to be unreliable at this - sometimes it doesn't seem to
create the link?

I've put this down to the fact that XP runs multiple XL books as independant
applications, and possibly dependant on how you open each book - but i don't
know for sure and i need this to work reliably

Anyone noticed this bug? if so, anyone know how to make it work reliably