Using ordinal numbers in chapter headings (e.g. First chapter)

Question

I'm writing a document using the book class. The normal chapter headings are of the form (for the first chapter in the book):

Chapter 1

<Name of first chapter>

Instead I would like it to be:

First chapter

< Name of first chapter>

and of course with "Second chapter" instead of "Chapter 2" and so forth. I don't have a lot of chapters, so if there is a method that involves manually entering the "First chapter" part, that would be acceptable. I think it would be necessary in fact, since the book is in Icelandic and I don't think Babel would support this sort of translation. So, for the record, I really want it to say "Fyrsti kafli" instead of "Chapter 1".

Ideally, this solution would also take care of the chapter name in the headers, but if not I can solve it by using

\markboth{First chapter. Name of first chapter}{1.1 Name of first subchapter}

Answer 1

One option using titlesec and fmtcount (to easily get the ordinal string for the chapter counter); using fancyhdr and a simple redefinition for \chaptermark gives the desired formatting for the header:

\documentclass{book}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{titlesec}
\usepackage{fmtcount}
\usepackage{fancyhdr}
\usepackage{lipsum}% just for the example

\titleformat{\chapter}[display]
  {\normalfont\huge\bfseries}
  {\Ordinalstring{chapter}~\chaptertitlename}
  {20pt}
  {\Huge}

\pagestyle{fancy}
\fancyhf{}
\fancyhead[OL]{\leftmark}
\fancyhead[ER]{\nouppercase\rightmark}
\renewcommand{\chaptermark}[1]{%
  \markboth{\Ordinalstring{chapter}~\chaptername.\ #1}{}}

\begin{document}

\chapter{A test chapter}
\section{Test section}
\lipsum[1-6]

\end{document} 

The result:

Since, as egreg notices in his comment, fmtcount doesn't support Icelandic, one can use an \ifcase:

\documentclass{book}
\usepackage[icelandic]{babel}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{titlesec}
\usepackage{fmtcount}
\usepackage{fancyhdr}
\usepackage{lipsum}% just for the example

\newcommand\icelandicordinal[1]{%
  \ifcase\value{#1}\or 
    Fyrsti\or 
    <second>\or...
    <third>%
  \fi
}

\titleformat{\chapter}[display]
  {\normalfont\huge\bfseries}
  {\icelandicordinal{chapter}~\MakeLowercase\chaptertitlename}
  {20pt}
  {\Huge}

\pagestyle{fancy}
\fancyhf{}
\fancyhead[OL]{\leftmark}
\fancyhead[ER]{\nouppercase\rightmark}
\renewcommand{\chaptermark}[1]{%
  \markboth{\icelandicordinal{chapter}~\MakeLowercase\chaptername.\ #1}{}}

\begin{document}

\chapter{A test chapter}
\section{Test section}
\lipsum[1-6]

\end{document} 

The result: