Let a package do the hard work for you (that's one of the reasons they were written in the first place). The rotating package offers you sidewaysfigure (and sidewaystable):
\documentclass{book}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{graphicx}
\usepackage{rotating}
\begin{document}
.\clearpage% just for the example
\begin{sidewaysfigure}
\centering
\includegraphics[width=8cm]{example-image-a}
\caption{rotated image}
\end{sidewaysfigure}
\begin{sidewaysfigure}
\centering
\includegraphics[width=8cm]{example-image-a}
\caption{rotated image}
\end{sidewaysfigure}
\end{document}

The code needs two runs to determine the right position.
From the package documentation:
If the twoside option has been given to the main document class
(either explicitly, or implicitly as in the default for book class),
the package will rotate sideways figures according to the page number
(this requires at least two passes through LaTeX). If you want the
twoside option, but want the figures always in one direction, use
the figuresright or figuresleft options to the package.
Now that I re-read the question, it seems that the orientation should be such that bottom of rotated figures should be near the spine; this is somehow against the accepted practice in which top of rotated figures should be near the spine; the rotating package doesn't implement an option for this situation, so some redefinitions are needed:
\documentclass{book}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{graphicx}
\usepackage{rotating}
\makeatletter
\def\end@rotfloat{%
\end{minipage}\end{lrbox}%
\stepcounter{r@tfl@t}%
\rot@label{RF\ther@tfl@t}%
\rot@pageref{RF\ther@tfl@t}{\R@@page}%
\edef\@tempa{Adding sideways \@captype\space on page \R@@page\space}
\rot@mess@toks\expandafter{\@tempa}
\wd\rot@float@box\z@
\ht\rot@float@box\z@
\dp\rot@float@box\z@
\vbox to \textheight{%
\setkeys{Grot}{units=360}%
\if@rot@twoside
\else
\let\R@@page\rot@LR
\fi
\ifthenelse{\isodd{\R@@page}}{%
\if@rot@twoside
\rot@mess@toks\expandafter{\the\rot@mess@toks (right hand page)}%
\fi
\@@line{%
\hskip\rotFPbot
\rotatebox{-90}{\box\rot@float@box}%
\hskip\rotFPtop
}%
\vfill
}{%
\if@rot@twoside
\rot@mess@toks\expandafter{\the\rot@mess@toks (left hand page)}%
\fi
\vfill
\@@line{%
\hskip\rotFPtop
\rotatebox{90}{\box\rot@float@box}%
\hskip\rotFPbot
}%
}%
\rot@message{\the\rot@mess@toks}
}%
\end@float
}
\let\endsidewaysfigure\end@rotfloat
\let\endsidewaystable\end@rotfloat
\makeatother
\begin{document}
.\clearpage% just for the example
\begin{sidewaysfigure}
\centering
\includegraphics[width=8cm]{example-image-a}
\caption{rotated image}
\end{sidewaysfigure}
\begin{sidewaysfigure}
\centering
\includegraphics[width=8cm]{example-image-a}
\caption{rotated image}
\end{sidewaysfigure}
\end{document}

figureor other floating environment or directly in the main text flow? – David Carlisle Jun 06 '13 at 21:43figureenvironment – Tim Jun 07 '13 at 06:33