Retrieve only chapter name in fancyhdr

There are three bits of information for each chapter:

1. \thechapter, which is the number. (For instance, 2 of "Chapter 2.")
2. \chaptername, which is the current language equivalent of the word "chapter."
3. And \chaptermark, the actual title of the chapter.

It looks like you're looking for \chaptermark. See also Section 9 of the fancyhdr documentation, especially Figure 3, which shows one way to do this:

\renewcommand{\chaptermark}[1]{%
\markboth{#1}{}}

Edit: After looking at this again, it looks like tufte-book doesn't even have chapter numbers. (This is where a MWE could have helped.) How's this?

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

\setcounter{secnumdepth}{0}

\pagestyle{fancy}
\fancyhf{}
\fancyhead[RO]{\textsc{\nouppercase{\newlinetospace{{\leftmark}}\quad\thepage}}}

\begin{document}

\chapter{Sample Title}
\lipsum[1-9]

\chapter{Another Sample Title}
\lipsum[1-9]

\chapter{And One More}
\lipsum[1-9]

\end{document}

In the end I integrated the answer given by Sam into the bits of code I was suggesting in my question. The following works:

\renewcommand{\chaptermark}[1]{%
\markboth{#1}{}}  
\fancyhf{} % clear header and footer fields
\ifthenelse{\boolean{@tufte@twoside}}
  {\fancyhead[LE]{\thepage\quad\smallcaps{\newlinetospace{\plaintitle}}}%
    \fancyhead[RO]{\smallcaps{\newlinetospace{\chaptername{}~\thechapter. \leftmark}}\quad\thepage}}
  {\fancyhead[RE,RO]{\smallcaps{\newlinetospace{\chaptername{}~\thechapter. \leftmark}}\quad\thepage}}

This gives the book title on left pages and the chapter title on right pages.

For some reason, \smallcaps works with this redefinition of chaptermark, which only gives the chapter title:

\renewcommand{\chaptermark}[1]{%
\markboth{#1}{}}

... and not with the following one, which yields the word chapter, followed by the chapter number and its full title:

\renewcommand{\chaptermark}[1]{%
\markboth{\MakeUppercase{%
\chaptername\ \thechapter.%
\ #1}}{}}