cover.tex

\documentclass{article}
\makeatletter
\usepackage{calc}
\newlength\onePageWidth
\newlength\onePageHeight
\newlength\spineWidth

% pagesize: 5.125in x 8.25in
% spinewidth: 9.374mm
\setlength{\onePageWidth}{5.125in}
\setlength{\onePageHeight}{8.25in}
\setlength{\spineWidth}{9.374mm}

\newlength\geoPaperWidth
\setlength\geoPaperWidth{2\onePageWidth+\spineWidth}
\newlength\frontCoverX
\setlength\frontCoverX{\onePageWidth+\spineWidth}

\usepackage[
  paperwidth=\geoPaperWidth,
  paperheight=\onePageHeight,
  nohead,
  nofoot,
  margin=0pt]{geometry}
\usepackage[cmyk]{xcolor}
\usepackage{graphicx}
\usepackage{eso-pic}
\graphicspath{{./}}
\makeatother

\begin{document}
\mbox{}
\pagecolor[RGB]{137, 101, 82}
%
\AddToShipoutPictureFG*{\put(\LenToUnit{0mm},\LenToUnit{0mm})%
{\includegraphics[keepaspectratio, height=\paperheight]{cover_back.pdf}}}
%
\AddToShipoutPictureFG*{\put(\LenToUnit{\onePageWidth},\LenToUnit{0mm})%
{\includegraphics[keepaspectratio, height=\paperheight]{cover_spine_L.pdf}}}
%
\AddToShipoutPictureFG*{\put(\LenToUnit{\frontCoverX},\LenToUnit{0mm})%
{\includegraphics[keepaspectratio, height=\paperheight]{cover_front.pdf}}}
%
\end{document}