thanks to Bernard's answer to my last question, I was able to obtain the ToC page appearance I wanted. Now I'm facing one last (hopefully) issue: the fact is that I thought that I could add some tracking (letter spacing) to my capitalized headings in any moment, and I didn't realize immediately that in Bernard's syntax it wasn't exactly straightforward. At least using the package soul as I had planned after some resarch here on tex.stackexchange. Here is my MWE:

\documentclass[11pt,twoside]{book}
\usepackage[a5paper,inner=14mm,outer=20mm,top=21mm,bottom=22.5mm]{geometry}

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{textcomp}
\usepackage{gensymb}
\usepackage{graphicx}
\usepackage{units}
\usepackage{tcolorbox}
\usepackage{fancyhdr}
\usepackage{microtype}
\usepackage{mwe}

\usepackage{soul}
\sodef\myspaced{}{.12em}{.6em plus.1em minus.1em}{.6em plus.1em minus.1em}

\usepackage{titlesec}
\usepackage[rightlabels]{titletoc}
\titlecontents{chapter}
[6.55em]
{\medskip}
{\hspace*{-6.55em}\textbf}
{\hfill\sffamily\contentspage}[\smallskip]

\titleformat{\chapter}[display]
{\normalfont\Large\filcenter\rmfamily}
{\LARGE\sffamily\MakeUppercase{\myspaced{\chaptertitlename}} \huge\thechapter}%  <--- HERE_1
{0.5pc}
{\color{green}\titleline*[c]{\titlerule[3pt]}\color{black}
\vspace{1pc}%
\Huge\bfseries}[\vspace{6pt}]
\setlength{\titlewidth}{0.65\textwidth}

\titleformat{name=\chapter, numberless}[block]
{\thispagestyle{empty}\vspace*{0.7ex}\normalfont\LARGE\filcenter\sffamily}%  <--- HERE_2
{}
{0em}
{\MakeUppercase}[\vspace{-0.8ex}{\color{green}\rule{\titlewidth}{3pt}}]%  <--- HERE_3

\begin{document}

\tableofcontents
\clearpage\thispagestyle{empty}

\chapter{Title of Chapter One}
\lipsum
\chapter{Title of Chapter Two}
\chapter{Title of Chapter Three is longer}
\chapter{Short}
\chapter{Other normal title}
\chapter{Other normal title}
\chapter{Other normal title}

\end{document}


my intervention at the line marked with HERE_1 let me obtain the chapter headings as I wanted. The following picture compares the outcome before and after introducing the "myspaced" command:

Now I'd like to get the same result with the word "CONTENTS" in the toc page:

But my attempts are failing. I've tried to introduce \myspaced in the lines HERE_2 and HERE_3, but I get (depending on where exactly I add it) from nothing, to affecting the sans serif-ness, to a compiling error. Can anybody help me work this out? Must I change package? Soul was so easy in its first application!

Thank you!

Here is a solution with microtype: I set the loading option [letterspace=100] (which means 100/1000 em). The package defines the command \textls{#1} and the switch \lsstyle.

\documentclass[11pt,twoside]{book}
\usepackage[a5paper,inner=14mm,outer=20mm,top=21mm,bottom=22.5mm]{geometry}

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{textcomp}
\usepackage{gensymb}
\usepackage{graphicx}
\usepackage{units}
\usepackage{tcolorbox}
\usepackage{fancyhdr}
\usepackage[letterspace=100]{microtype}
\usepackage{mwe}

\usepackage{titlesec}
\usepackage[rightlabels]{titletoc}
\titlecontents{chapter}
[6.55em]
{\medskip}
{\hspace*{-6.55em}\textbf}
{\hfill\sffamily\contentspage}[\smallskip]

\titleformat{\chapter}[display]
{\normalfont\Large\filcenter\rmfamily}
{\LARGE\sffamily\MakeUppercase{\textls{\chaptertitlename}} \huge\thechapter}% <--- HERE_1
{0.5pc}
{\color{green}\titleline*[c]{\titlerule[3pt]}\color{black}
\vspace{1pc}%
\Huge\bfseries}[\vspace{6pt}]
\setlength{\titlewidth}{0.65\textwidth}

\titleformat{name=\chapter, numberless}[block]
{\thispagestyle{empty}\vspace*{0.7ex}\normalfont\LARGE\filcenter\sffamily}% <--- HERE_2
{}
{0em}
{\lsstyle\MakeUppercase}[\vspace{-0.8ex}{\color{green}\rule{\titlewidth}{3pt}}]% <--- HERE_3

\begin{document}

\tableofcontents
\clearpage\thispagestyle{empty}

\chapter{Title of Chapter One}
\lipsum
\chapter{Title of Chapter Two}
\chapter{Title of Chapter Three is longer}
\chapter{Short}
\chapter{Other normal title}
\chapter{Other normal title}
\chapter{Other normal title}

\end{document}


• Wow, with hindsight it seems so easy, microtype was already there... really an Egg of Columbus! Thank you Bernard, you saved me so much time!! – lesath82 Oct 26 '17 at 23:14