How can I design a book cover?

Here is a poor man's solution! Cover and title pages should ideally be designed by a graphics artist, as most of us are really challenged in this area! (If you have not watched this brilliant TED Talk by Book Cover Designer Chip Kidd, I advise you to watch it as you can gain a lot of insight into book cover design).

The code follows:

\documentclass{book}
\usepackage{graphicx}
%\usepackage{fancyvrb}
\begin{document}
\clearpage
%% temporary titles
% command to provide stretchy vertical space in proportion
\newcommand\nbvspace[1][3]{\vspace*{\stretch{#1}}}
% allow some slack to avoid under/overfull boxes
\newcommand\nbstretchyspace{\spaceskip0.5em plus 0.25em minus 0.25em}
% To improve spacing on titlepages
\newcommand{\nbtitlestretch}{\spaceskip0.6em}
\pagestyle{empty}
\begin{center}
\bfseries
\nbvspace[1]
\Huge
{\nbtitlestretch\huge
AWK ONE LINERS EXPLAINED}

\nbvspace[1]
\normalsize

TO WHICH IS ADDED MANY USEFUL ONE\\
LINERS AND CODE SO THAT\\
YOU CAN AWK LIKE A HAWK
\nbvspace[1]
\small BY\\
\Large PETERIS KRUMINS\\[0.5em]
\footnotesize AUTHOR OF ``A WORKING ALGEBRA,'' ``WIRELESS TELEGRAPHY,\\
ITS HISTORY, THEORY AND PRACTICE,'' ETC., ETC.

\nbvspace[2]

\includegraphics[width=1.5in]{./graphics/pic37}
\nbvspace[3]
\normalsize

DOHA\\
\large
PUBLISHED IN THE WILD
\nbvspace[1]
\end{center}
\end{document}

Create an own document (1 or 4 pages) for your cover and then merge it with the other document with the package pdfpages. Examples of book covers.

For more detailed workaround, see Creating Book Covers using PSTricks. You will be guided how to create a book cover as shown above.

For my own cover I use a template:

On this I put all text frames and additional images with \rput from PSTricks or \put from standard LaTeX.

You can use xcoffins package from LaTeX 3 project. This example is inspired by work of Jan Tschichold

\documentclass{article}

\usepackage[a4paper,margin=5pt]{geometry}

\usepackage[T1]{fontenc}
\usepackage{times,multicol,graphicx}
\usepackage{xcoffins}
\usepackage{times,color}
\newcommand\cbox[2][.8]{{\setlength\fboxsep{0pt}\colorbox[gray]{#1}{#2}}}
\pagestyle{empty}
\begin{document}
  \NewCoffin \result
  \NewCoffin \aaa
  \NewCoffin \bbb
  \NewCoffin \ccc
  \NewCoffin \ddd
  \NewCoffin \eee
  \NewCoffin \fff
  \NewCoffin \rulei
  \NewCoffin \ruleii
  \NewCoffin \ruleiii

\SetHorizontalCoffin \result {}
\SetHorizontalCoffin \aaa {\fontsize{52}{50}\sffamily\bfseries mitteilungen}
\SetHorizontalCoffin \bbb {\fontsize{52}{50}\sffamily\bfseries typographische}
\SetHorizontalCoffin \ccc {\fontsize{12}{10}\sffamily 
                      \quad zeitschrift des bildungsverbandes der
                      deutschen buchdrucker leipzig 
                     \textbullet{} oktoberheft 1925}
\SetHorizontalCoffin \ddd {\fontsize{28}{20}\sffamily sonderheft}
\SetVerticalCoffin \eee {180pt}
                 {\raggedleft\fontsize{31}{36}\sffamily\bfseries 
                      elementare\\
                      typographie}
\SetVerticalCoffin \fff {140pt}
                 {\raggedright \fontsize{13}{14}\sffamily\bfseries 
                       natan altman \\
                       otto baumberger \\
                       herbert mayer \\
                       max burchartz \\
                       el lissitzky \\
                       ladislaus moholy-nagy \\
                       moln\'ar f.~farkas \\
                       johannes molzahn \\
                       kurt schwitters \\
                       mart stam \\
                       ivan tschichold}

\RotateCoffin \bbb {90}
\RotateCoffin \ccc {270}

\SetHorizontalCoffin \rulei  {\color{red}\rule{6.5in}{1pc}}
\SetHorizontalCoffin \ruleii {\color{red}\rule{1pc}{23.5cm}}
\SetHorizontalCoffin \ruleiii{\color{black}\rule{10pt}{152pt}}


\JoinCoffins \result                \aaa 
\JoinCoffins \result[\aaa-t,\aaa-r] \rulei   [b,r](0pt,2mm)
\JoinCoffins \result[\aaa-b,\aaa-l] \bbb     [B,r](2pt,0pt)
\JoinCoffins \result[\bbb-t,\bbb-r] \ruleii  [t,r](-2mm,0pt)
\JoinCoffins \result[\aaa-B,\aaa-r] \ccc     [B,l](66pt,14pc)
\JoinCoffins \result[\bbb-l,\ccc-B] \fff     [t,r](-2mm,0pt)
\JoinCoffins \result[\fff-b,\fff-r] \ruleiii [b,l](2mm,0pt)
\JoinCoffins \result[\ccc-r,\fff-l] \eee     [B,r]
\JoinCoffins \result[\eee-T,\eee-r] \ddd     [B,r](0pt,4pc)
\TypesetCoffin \result
\end{document}