System/System spacing

[Adrian Oehm]

Hi

This is probably a dumb question, but can someone please tell me where to find 
info/examples on (or explain how to) increasing the system/system spacing.

I have a piece with 3 systems which looks bunched on the page - I'd like to 
separate them a little more (the piece is part of a book, set in a bookpart).

Any pointers would be great (I've been scrolling through the manuals all 
afternoon and can't seem to find something that works...)

Thanks

Adrian