documentclass{memoir}
renewcommand*{thebook}{Alph{book}} % "Book A", "Book B", etc.
begin{document}
book{maindoc}
chapter{maindoc first chapter}
label{chap:maindoc-first}
chapter{maindoc second chapter}
label{chap:maindoc-second}

book{extradoc}
setcounter{chapter}{0} % reset chapter counter with each book
chapter{extradoc first chapter}
label{chap:extradoc-first}
chapter{extradoc second chapter}
label{chap:extradoc-second}
As shown in ref{chap:maindoc-first}, and further research in ref{chap:extradoc-first} ...

book{anotherdoc}
setcounter{chapter}{0}
chapter{anotherdoc chapter}
end{document}