make customEps path resolution more robust

private_includes/book/book_include.ily

@@ -205,7 +205,7 @@ songs =
               \bookpart {
                 $header
                 \headerToTOC #header #label
-                \score { $music \layout { $layout } }
+                \score { $music \layout { $layout songfilename = $(symbol->string filename) } }
                 $(add-text-pages text-pages)
               }
             #}))))))