LaTeX: Thumbs with fancyhdr

%%% Source:
%%% Tobi in http://tex.stackexchange.com/a/57317/9237
%%% Some changes by Speravir
%%%
%%% Compare http://pastebin.com/EKr5CTjN

\documentclass{book}

% load TikZ to draw the boxes
\usepackage{tikz}
\usetikzlibrary{calc}

% use fancyhdr or whatever you want to add
% the boxes to the header to make them appear
% on every page
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand*{\headrulewidth}{0pt}

% new counter to hold the current number of the
% letter to determine the vertical position
\newcounter{letternum}
% newcounter for the sum of all letters to get
% the right height of a box
\newcounter{lettersum}
\setcounter{lettersum}{26}
% some margin settings
\newlength{\thumbtopmargin}
\setlength{\thumbtopmargin}{1cm}
\newlength{\thumbbottommargin}
\setlength{\thumbbottommargin}{3cm}
% calculate the box height by dividing the page height
\newlength{\thumbheight}
\pgfmathsetlength{\thumbheight}{%
    (\paperheight-\thumbtopmargin-\thumbbottommargin)%
    /%
    \value{lettersum}
}
% box width
\newlength{\thumbwidth}
\setlength{\thumbwidth}{1.5cm}
% style the boxes
\tikzset{
    thumb/.style={
        fill=black!50!red,
        text=white,
        minimum height=\thumbheight,
        text width=\thumbwidth,
        outer sep=0pt,
        font=\sffamily\bfseries,
    }
}
\newcommand{\oddthumb}[1]{%
        % see pgfmanual.pdf for more information about this part
        \begin{tikzpicture}[remember picture, overlay]
            \node [thumb,align=right,anchor=north east,] at ($%
                (current page.north east)-%
                (0,\thumbtopmargin+\value{letternum}*\thumbheight)%
            $) {#1};
        \end{tikzpicture}
}
\newcommand{\eventhumb}[1]{%
        % see pgfmanual.pdf for more information about this part
        \begin{tikzpicture}[remember picture, overlay]
            \node [thumb,align=left,anchor=north west,] at ($%
                (current page.north west)-%
                (0,\thumbtopmargin+\value{letternum}*\thumbheight)%
            $) {#1};
        \end{tikzpicture}
}
% create a new command to set a new lettergroup
\newcommand{\lettergroup}[1]{%
\fancypagestyle{chapterstart}{%
\fancyhf{}
\chead{\oddthumb{#1}}% chapters start only on odd pages
\cfoot{\thepage}
}
    % add a chapter (optional)
    \chapter*{#1}\thispagestyle{chapterstart}%
%     \section*{#1}
    % use one head or foot element to put in the box
    % it doesn't matter which you use since the box
    % is positioned on the page absolutely
    \fancyhead[LO]{\oddthumb{#1}}%
    \fancyhead[RE]{\eventhumb{#1}}%
    % step the counter of the letters
    \stepcounter{letternum}%
}

% for some blindtext
\usepackage{lipsum}

\begin{document}

% usage: \lettergroup{<letter>}
% e.g.
\lettergroup{A}
% your text
\lipsum[1-20]

\lettergroup{B}
\lipsum[21-40]

\lettergroup{C}
\lipsum[41-60]

\lettergroup{D}
\lipsum[61-80]
\end{document}