make customEps path resolution more robust

private_includes/base/eps_file_from_song_dir.ily

@@ -6,10 +6,20 @@
 #(define-markup-command (customEps layout props ysize filename)(number? string?)
    #:properties ((songfilename ""))
    (interpret-markup layout props
-     (let ((defaulttitlemarkup (ly:output-def-lookup layout 'defaultTitleMarkup))
-           (filepath (if (string-null? songfilename)
-                          filename
-                          (ly:format "~a/~a/~a" songPath songfilename filename))))
+     (let* ((defaulttitlemarkup (ly:output-def-lookup layout 'defaultTitleMarkup))
+            ;; Determine the song directory to resolve the EPS relative to.
+            ;; In TEXT the name arrives via the markup props (\setsongfilename),
+            ;; but inside MUSIC (e.g. \mark) the props do not carry it, so we
+            ;; fall back to the value stored in the score's \layout block.
+            (layout-songfilename (ly:output-def-lookup layout 'songfilename #f))
+            (songdir (cond ((not (string-null? songfilename)) songfilename)
+                           ((and (string? layout-songfilename)
+                                 (not (string-null? layout-songfilename)))
+                            layout-songfilename)
+                           (else #f)))
+            (filepath (if songdir
+                          (ly:format "~a/~a/~a" songPath songdir filename)
+                          filename)))
        (if (file-exists? filepath)
              (make-epsfile-markup Y ysize filepath)
              (if defaulttitlemarkup

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)
               }
             #}))))))