\version "2.21.0"

% Define all the music first
start = \relative { \repeat volta 10 { c'4 d e f } }
endA  = \relative { g'2 a4 b c1 }
endB  = \relative { g'2 f4 d c1 }
music = #(define-music-function () () #{ \start \end \bar "|." #})

% book template
mybookdef = "
  \book{
    \bookOutputSuffix \bookName
    \paper{
      system-count = \mysyscount
    }
    \score {
      \unfoldRepeats \repList { \music }
      \layout{ } 
    } 
  }
"

mybook = #(define-scheme-function () () 
          (ly:parser-include-string #{ \mybookdef #}))

% "none" is not a valid 1st argument to \unfoldRepeats,
% but the default action (ignore \unfoldRepeats) is what we want ;-)
repList = none
bookName = "A" end = \endA mysyscount = 1 \mybook
bookName = "B" end = \endB mysyscount = 1 \mybook
repList = volta
bookName = "C" end = \endA mysyscount = 2 \mybook
bookName = "D" end = \endB mysyscount = 3 \mybook