Page break table with single cell

Question

I have an environment set up of longtables that I use as "tips"-boxes and exercises in my book. The left column shows only one word rotated (tips, exercise or whatever) while the right contains the rest of the text. Now the problem is that the right column is a single cell so (xe)latex decides to put the table on a new page if it is too big. Here is an example of what I mean:

So how can I make it start on the current page and break whenever necessary, then continue on the next page?

My code for the table:

\documentclass[11pt]{book}

\usepackage{longtable}
\usepackage{array}
\usepackage{rotating}
\usepackage{lipsum}

\newcolumntype{M}[1]{>{%
  \parskip=0.5\baselineskip%
  \advance\parskip by 0pt plus 2pt
   \setlength{\parfillskip}{30pt plus 1fil}}m{#1}}

\newenvironment{exercise}[1]
{
    \begin{longtable}{m{0.05\textwidth}|M{0.85\textwidth}}
    \rotatebox{90}{\textbf{Övning}} & \textbf{#1} \newline
}
{\end{longtable}
\bigskip
}

\begin{document}

\lipsum

\begin{exercise}{1}
\lipsum
\end{exercise}

\end{document}

Answer 1

You can't use longtable is such a combination. In the comment of cmhughes the package mdframed was recommended. In the example below you can find two possibilities with mdframed.

If you have further questions you can ask ;-)

EDIT: CHANGED CODE

\documentclass[11pt]{book}
\usepackage[framemethod=default]{mdframed}
\usepackage{rotating}

\usepackage{lipsum}
\makeatletter


\newrobustcmd\Titleenv[1]{%
   \setbox0=\hbox{\rotatebox[origin=cc]{90}{\textbf{#1}}}
   \setlength\@tempdimc{\dimexpr\ht0+\dp0\relax}
   \renewcommand*\md@@frametitle{%
      \llap{\smash{\raisebox{-.5\@tempdimc}{\box0}\hspace*{2em}}}}%
   \appto\md@frame@leftline@single{%
     \ifdimgreater{\mdfboundingboxheight}{\@tempdimc}%
     {}%
     {%
       \llap{\color{\mdf@middlelinecolor}%
           \rule[\dimexpr-.5\@tempdimc-\mdf@innerbottommargin@length\relax]%
                {\mdf@middlelinewidth@length}%
                {\dimexpr\@tempdimc+\mdf@innerbottommargin@length
                         +\mdf@innertopmargin@length\relax}%
            }%
     }%
   }%
}

\newenvironment{exercise}{%
  \mdfsetup{topline=false,rightline=false,bottomline=false,linecolor=red,
          linewidth=2pt,skipbelow=\topskip,skipabove=\topskip}
   \Titleenv{\"Ovning}%
\begin{mdframed}[]%
}{\end{mdframed}}

\begin{document}
\begin{exercise}
\lipsum[1]
\end{exercise}

\lipsum[1]

\begin{exercise}
Text
\end{exercise}

\lipsum[1]

\begin{exercise}
\lipsum
\end{exercise}
\end{document}

The code above results in the following picture:

Answer 2

It is also possible to have the line and vertical text inside the default text width

\documentclass[11pt]{book}
\usepackage{framed}
\usepackage{rotating}
\usepackage{lipsum}   
\newenvironment{exercise}[1]
  {\par\begin{leftbar}\noindent\textbf{#1}\\
   \makebox(0,0){\put(-30,-50){\rotatebox{90}{\textbf{Övning}}}}\ignorespaces}
  {\end{leftbar}\par}

\begin{document}    
\lipsum

\begin{exercise}{1}
\lipsum
\end{exercise}

\end{document}