Score identifier not working in book block

[Michael Brennan]

Hello,
I wonder why this does not work in lilypond (version 2.8.0):

myscore = \score {
 \relative c' { c d e }
}
\book {
 \myscore
}

I get a syntax error, unexpected SCORE_IDENTIFIER
 \myscore

If I put the \score directly in \book it works,
but why doesn't it work with the identifier?

Thanks!
/Michael Brennan