space-between-pieces

[David Bobroff]

I'm trying to put extra space between consecutive pieces in a 
\bookpart{} block and have been unsuccessful.  I have looked carefully 
through "4.1.2 Page formatting: Vertical dimensions" and tried a number 
of things, but none of them seems to apply to this situation.  The only 
thing that has done anything to increase the space between pieces is 
inserting:

\markup{ " " }

...between their \score{} blocks.  This helps some, but I'd like a bit 
more space and I'd like to be able to control the amount of space as 
well.  Help?

-David