Adding ``Chapter N'' to all chapters but the first one

Question

Building on this example where Chapter is added to each chapter heading, how can I modify the code to do just that except for the introduction and final / concluding chapter (but still retain numbering for TOC purposes)?

\documentclass{tufte-book}
\usepackage{lipsum}

\makeatletter
\titleformat{\chapter}%
  [display]% shape
  {\relax\ifthenelse{\NOT\boolean{@tufte@symmetric}}{\begin{fullwidth}}{}}% format applied to label+text
  {\itshape\huge\chaptertitlename~\thechapter}% label
  {0pt}% horizontal separation between label and title body
  {\huge\rmfamily\itshape}% before the title body
  [\ifthenelse{\NOT\boolean{@tufte@symmetric}}{\end{fullwidth}}{}]% after the title body
\makeatother
\setcounter{secnumdepth}{1}

\begin{document}

\chapter{Introduction}
\lipsum[1]
\chapter{A substantive chapter}
\lipsum[2]
\end{document}

Edit: I am interested in a flexible solution that - allows me to keep the introduction and conclusion as numbered the same way as the other chapters - allows me to have the intro and conclusion not numbered, but still to appear in the TOC.

Answer 1

As long as the changes are made inside a group, they only affect the chapter you want and you are flexible to change the other chapters however you want.

\documentclass{tufte-book}
\usepackage{lipsum}

\makeatletter
\newcommand{\mychap}{%
\titleformat{\chapter}%
  [display]% shape
  {\relax\ifthenelse{\NOT\boolean{@tufte@symmetric}}{\begin{fullwidth}}{}}% format applied to label+text
  {\itshape\huge\chaptertitlename~\thechapter}% label
  {0pt}% horizontal separation between label and title body
  {\huge\rmfamily\itshape}% before the title body
  [\ifthenelse{\NOT\boolean{@tufte@symmetric}}{\end{fullwidth}}{}]% after the title body
}
\makeatother
\setcounter{secnumdepth}{1}

\begin{document}

\chapter{Introduction}
\lipsum[1]

\begingroup
\mychap%
\chapter{A substantive chapter}
\lipsum[2]
\chapter{Another chapter}
\lipsum[1]
\endgroup

\chapter{Conclusions}
\lipsum[1]

\end{document}