\book {...}
secions in the file. Each such block will be converted into its own PDF, named filename-1.pdf, filename-2.pdf, etc. The \book {..}
block typically looks like (note that the \header section needs to come after the score and after a staff, otherwise lilypond will fail with an error message...)
\book { \score { \new Staff << \va >> } \header {instrument = "Viola"} }
To have some control about the naming of the output files, you can use
#(define output-suffix "VcB")to define a suffix for the filename. The number will still be added, so the filename for the \book(s) after this definition will be filename-VcB-3.pdf, etc.