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   \begin{longtabu} to \textwidth {X[l]X[l,1]X[l]}
      \caption{YYY\label{tab:YYY}} \\\toprule
      \textbf{YYY}  &
      \textbf{YYY} &
              \textbf{YYY} \\
...

\documentclass [a4paper, 12pt, openany,	headsepline, footsepline, twoside, listof=totoc, titlepage, bibliography=totoc] {scrbook}

\usepackage[ngerman]{babel}			
\usepackage[T1]{fontenc}				
\usepackage{lmodern}						
\usepackage{amsmath}						
\usepackage{amsfonts}				
\usepackage{amssymb}						
\usepackage[text={15cm,24cm},left=3cm,top=2cm]{geometry}						
\usepackage{graphicx} 					
\usepackage{float}							
\usepackage{scrhack}						
\usepackage{tabularx}					
\usepackage{longtable}					
\usepackage{supertabular}
\usepackage{booktabs}
\usepackage[hang,small,bf]{caption}		
\usepackage{color}							
\usepackage{cite}								
\usepackage[latin1]{inputenc}		
\setlength{\parindent}{0pt}			
\usepackage{totpages}						
\usepackage{sectsty}						
\usepackage[automark]{scrpage2} 
\usepackage[thinspace,thinqspace,squaren,textstyle]{SIunits} 
\usepackage{mathptmx}		
\usepackage[doublespacing]{setspace}
\usepackage{textcomp} 
\usepackage{ragged2e} 
\usepackage{longtable, tabu,  booktabs} 
\pagestyle{useheadings}		
\setlength{\headheight}{1.1\baselineskip}																			

\begin{document}

\begin{singlespacing}
	\begin{longtabu} to \textwidth {X[l]X[l,1]X[l]}
		\caption{YYY} \\
		\toprule
		\label{tab:YYY}
		\textbf{\textup{YYY}}  &
		\textbf{YYY} &
  	         \textbf{YYY} \\
  	\midrule\midrule
		\endfirsthead
		\textbf{\textup{YYY}} &
		\textbf{YYY} &
  	        \textbf{YYY} \\
  	\midrule
  	\endhead
		\midrule
		\multicolumn{3}{r}{\emph{- Fortsetzung auf der nächsten Seite -}}
		\endfoot
		\midrule\bottomrule
		\endlastfoot
  	XXX&XXX&XXX\\ \midrule
	XXX&XXX&XXX\\ \midrule
        XXX&XXX&XXX\\ \midrule
        XXX&XXX&XXX\\ \midrule
	\end{longtabu}
\end{singlespacing}

\textup{Überschrift}