Rather than copying the definition from book, you have to supply the forgotten one given by titlesec
\documentclass{tufte-book}\makeatletter\renewcommand{\subsubsection}{\ttl@straightclass{subsubsection}}\makeatother\titleformat{\subsubsection}[hang] {\fontsize{36}{42}\selectfont} {\thesubsubsection} {1em} {} []\setcounter{secnumdepth}{2}\begin{document}\chapter{A}\section{B}\subsection{bar}\subsubsection{foo}\paragraph{C}\end{document}Image may be NSFW.
Clik here to view.
