documentclass[b5paper]{book}
usepackage[final]{pdfpages}
usepackage{tikz}
usepackage{ifthen}

usepackage{lipsum}

begin{document}

tableofcontents

chapter{First Chapter}
lipsum lipsum


newpage
ifoddvalue{page}
  AddToShipoutPictureBG{
      begin{tikzpicture}[remember picture,overlay]
      node[minimum width=2cm, minimum height=2cm, font=Huge, rotate=90,
        rectangle, opacity=1, fill=black, text=white]
        at (current page.north east) {I};
        end{tikzpicture}
 }
fi

% Some pdf with several pages
%includepdf[pages=-,pagecommand={},width=textwidth]{1.pdf}
lipsum

newpage
ifoddvalue{page}
  AddToShipoutPictureBG{
      begin{tikzpicture}[remember picture,overlay]
      node[minimum width=2cm, minimum height=2cm, font=Huge, rotate=90,
        rectangle, opacity=1, fill=black, text=white]
        at (current page.north east) {II};
        end{tikzpicture}
 }
fi
%includepdf[pages=-,pagecommand={},width=textwidth]{2.pdf}
lipsum
end{document}