\version "2.19.45"

makeBookpart =
#(define-scheme-function ()()
   #{
     \bookpart {
       \score {
         \new Staff { d' }
       }
     }
   #})

#(let*
  ((book
    #{
      \book {

        \bookpart {
          \markup "This will be a title page"
        }

        \include "bookpart.ily"
        \include "bookpart.ily"

        %\makeBookpart

      }
    #}))
  (ly:book-process
   book
   #{ \paper {} #}  ; non-functional, placeholder
   #{ \layout {} #} ; non-functional, placeholder
   (ly:parser-output-name)))