Importing / inserting files into a single book using \bookpart

[Dirck Nagy]

Dear Lilypond,

I am trying to engrave a "book" of pieces by a single composer.  I was hoping 
that using \bookpart would make controlling print layout easier.

I have already engraved each individual piece, each in their own separate file.

I am unable to combine these files using \bookpart.  I don't know which parts 
to cut-and-paste.

I entered a few random notes using the \bookpart examples from the manual, and 
while very simple experiments work, I cannot insert anything more complicated 
from another file.

Perhaps something is wrong with my template?

Could someone please show me where and how to insert the file "Stripped Down 
Template" into the "Book Title Experiment" file?   Each file engraves on its 
own, and pasting the "template" file at the end of "Book" will work, but I 
cannot paste it into a previous page, nor can I add another \bookpart after it 
for subsequent files.

If you have the time, of course!

Thanks in advance!
dirck

Code is enclosed below.  "Book Title Experiment" is in red.

\version "2.24.1"


#(set-default-paper-size "letter")


  \header {
    title = "Book title"
    copyright = "Copyright line on book first page"

  }

  \bookpart {

    \header {
      subtitle = "First part" }
    \markup { The first book part }


  }

  \bookpart {
    \header { title = "new book"  subtitle = "Second part" }
    { c'4 }
  }

  \bookpart {
    \header { subtitle = "third part" }
    { c8 c16 c }

  }



\version "2.24.1"


\header {
  title = "Stripped Down template"
  subtitle = "subtitle"
  composer = "composer"
}

global = {
  \key d \major
  \time 3/4
  \tempo "tempo"
}

VoiceOne = \relative c' {
  \global

  f4 f
}

VoiceTwo = \relative c' {
  \global
   \stemDown

  d4 d
}

VoiceThree = \relative c' {
  \global
}

VoiceFour = \relative c' {
  \global
}

SpacingVoice = {
}

\score {
  \new Staff  { \clef "treble_8" << \VoiceOne \\ \VoiceTwo \\ \VoiceThree \\ 
\VoiceFour \\ \SpacingVoice >> }

}