I have employed fancyhdr to modify the headers of the chapters and sections to fit my taste. They are now written in lower case and alternate on even and odd pages:
\documentclass[a4paper]{book}
\usepackage[finnish]{babel}
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\sectionmark}[1]{\markright{\thesection\ #1}}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\fancyhead{} % clear all header fields
\fancyfoot{} % clear all footer fields.
\fancyhead[LE]{\textsl{\leftmark}} % L = left; E = even pages
\fancyhead[RO]{\textsl{\rightmark}} % R = right; O = odd pages
\fancyhead[RE]{\thepage} % R = right; E = even pages
\fancyhead[LO]{\thepage} % L = left; O = odd pages
\begin{document}
\tableofcontents
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\chapter{Xxx}
\section{Yyy}
\section{Yyy}
\section{Yyy}
\end{document}
The headers on the table of contents pages are still in uppercase. How can I extend my mofifications to apply the table of contents pages in full as well? I have included the call to babel in case it matters.