# reducing space between sections

I want to reduce space between between specified sections sections (C and D) only in my document. I don' want disturb my rest document. Here is a minimum code

    \documentclass[11pt]{book}              % Book class in 11 points
\parindent0pt  \parskip10pt             % make block paragraphs
\raggedright                            % do not right justify

\title{\bf An Example of Book Class}    % Supply information

\begin{document}
\frontmatter
\maketitle
\tableofcontents
\mainmatter
The following sectioning commands are available:

• You are aware that this depends on the documentclass most likely? – user31729 Oct 20 '15 at 17:44
• try \newcommand\addvspace{\vspace{-1em}} in preambul and \addvspace\section{A Subheading D} – touhami Oct 20 '15 at 18:15
• @touhami: Isn't \addvspace not already a core command? – user31729 Oct 20 '15 at 18:30
• @ChristianHupfer I mean \mtaddvspace, sorry. – touhami Oct 20 '15 at 18:32