Zusätzliche Informationen für subsection im Header von beamer

\documentclass[12pt]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{refcount}% Wenn man die auskommentierte Zeile mit \getrefnumber verwenden will.

\usetheme{AnnArbor}

\makeatletter
\newcommand*{\additionalinfoof}[1]{%
  \expandafter\ifx\csname additionalinfo@#1\endcsname\relax\else
    \expandafter\ifx\csname additionalinfo@#1\endcsname\@empty\else
      : \@nameuse{additionalinfo@#1}%
  \fi\fi
}
\newcommand*{\additionalinfo}[2][\thesection.\thesubsection]{%
  \global\@namedef{additionalinfo@#1}{#2}%
}
\makeatother

\usepackage{xpatch}
\makeatletter
\xpatchcmd\beamer@subsectionintoc{#3}{#3\additionalinfoof{#1.#2}}{}{\FailED}
\makeatother

\setbeamertemplate{headline}% Definition aus beamerouterthemeinfolines.sty geändert
{
  \leavevmode%
  \hbox{%
  \begin{beamercolorbox}[wd=.5\paperwidth,ht=2.65ex,dp=1.5ex,right]{section in head/foot}%
    \usebeamerfont{section in head/foot}\insertsectionhead\hspace*{2ex}
  \end{beamercolorbox}%
  \begin{beamercolorbox}[wd=.5\paperwidth,ht=2.65ex,dp=1.5ex,left]{subsection in head/foot}%
    \usebeamerfont{subsection in head/foot}\hspace*{2ex}\insertsubsectionhead\additionalinfoof{\thesection.\thesubsection}
  \end{beamercolorbox}}%
  \vskip0pt%
}

\begin{document}

\section{Thema A}

\begin{frame}
  \frametitle{Gesamtübersicht zum Thema A}
  \tableofcontents
\end{frame}

\subsection{Unterthema 1}
\begin{frame}
  \frametitle{Übersicht zu Unterthema 1}
  \additionalinfo{siehe Buch S. 8 und Paper XYZ}
  \tableofcontents[subsectionstyle=show/shaded]
\end{frame}
\begin{frame}
  \frametitle{Inhalt1}
  Inhalt1
\end{frame}
\begin{frame}
  \frametitle{Inhalt2}
  Inhalt2
\end{frame}

\subsection{Unterthema 2}
\begin{frame}
  \frametitle{Übersicht zu Unterthema 2}
  \additionalinfo{siehe Buch S. 95ff}
  \tableofcontents[subsectionstyle=show/shaded]
\end{frame}
\begin{frame}
  \frametitle{Inhalt1}
  Inhalt1
\end{frame}
\begin{frame}
  \frametitle{Inhalt2}
  Inhalt2
\end{frame}

\end{document}