The errors seem to have been produced by wrong symlinks, perhaps the
result of having both lilypond 2.2.6 and lilypond-snapshot 2.4.2 installed.
The symlink for lilypond is in /usr/share/texmf/tex/lilypond and points
The tex output of lilypond-book-snapshot uses this symlink and not (as
could be expected) /usr/share/texmf/tex/lilypond-snapshot which would
point to /usr/share/lilypond/2.4.2/tex.
I have now changed the symlink so that /usr/share/texmf/tex/lilypond
points to /usr/share/lilypond/2.4.2/tex and now lilypond-book-snapshot
works perfectly again! But I haven't now dared to try lilypond-book again.
Perhaps this is a bug which could be rectified?