variables in \book { }

[Graham King]

I'm working on a book containing twelve short single-movement quintets, originally with the intention of assembling them into a single volume with book-titling.ily.  However, being unable to make book-titling.ily deliver a ragged-last-bottom margin for each of the twelve quintets, I'm now trying to migrate to using \book { \score { ... } \score ... }.

Here's the problem:

>     \version "2.19.35"
>     \book {
>       music = \relative { c' d e f }
>       \score {
>         \music
>       }
>     }
fails with:
>     book_test.ly:4:3: error: syntax error, unexpected STRING  
>       music = \relative { c' d e f }
>     book_test.ly:6:5: error: unknown escaped string: `\music'    
>         \music
>     /book_test.ly:6:5: error: unrecognized string, not in text script or \lyricmode    
>         \music
whereas
>     \version "2.19.35"
>     music = \relative { c' d e f }  
>     \book {
>       \score {
>         \music
>       }
>     }
works fine.

To avoid having to restructure a number of files, it would be very handy to be able to use the former layout (defining music = { ... }, and a number of other variables, within \book but outside \score).  Is there a way to achieve that? 
(Alternatively, if there's a way to make book-titling.ily do a ragged-last-bottom for each piece, that would save even more work).

hopefully,
-- Graham