how would one cross-reference two [or more] books?

[Kieren MacMillan]

Hi all,

A very large part of my composition [and thus Lilypond engraving] career is 
concerned with very large, multi-format musical dramas (operas, musicals, 
plays-with-music, dramatic song cycles, etc.). A typical set of outputs might 
be:

   – full orchestra/band score
   – individual orchestra/band parts
   – Piano/Conductor score (reduction/arrangement)
   – Piano/Vocal score (a *different* reduction/arrangement)
   – Vocal/Choral score (reduction/arrangement)
   – libretto

I would *love* to be able to have a note in the (e.g.) P/C score that says 
“This song is found on pg. 12 of the Vocal/Choral score” — at the very least at 
the song level, but optimally at any arbitrary point in the score (e.g., top of 
every page, beginning of a given section of a song, etc.).

1. What mechanism(s) would be best used to accomplish this?

2. Can it be done [relatively easily] if each of the outputs are driven from an 
individual .ly file, or would it really require all outputs to be coded in the 
same .ly file [with separate \book blocks]?

Thanks!
Kieren.