This answer is actually a comment on Werner's answer. Although initially I accepted and used Tobi's answer, in the end I decided to use the method described in Werner's answer.
I noticed some interesting things I'd like to share. It all started when I decided to leave chapter titles (Chapter 1,2...) bold but remove boldface from chapter names (Introduction, ...).
Here is an excerpt from book.cls
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\huge\bfseries \@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
The first \bfseries refers to chapter title (Chapter 1,2...) and the second to chapter name.
So:
If I simultaneously use
\patchcmd{\@makechapterhead}{\bfseries}{}{}{}
\patchcmd{\@makechapterhead}{\bfseries}{}{}{}
neither chapter title nor name should be bold. And indeed this is what happens.
If I use only
\patchcmd{\@makechapterhead}{\huge\bfseries }{\huge }
the chapter title shouldn't be boldface but the chapter name should be. And indeed this is what happens.
BUT
If I use
\patchcmd{\@makechapterhead}{\Huge \bfseries}{\Huge }{}{}
I expect the chapter title to be boldface and the chapter name to be non-boldface. Instead both elements are boldface!!!!
Somehow the first (refering to chapter title) \bfseries also affects the chapter name (I don't know why but I guess people that know more about latex syntax could explain).
In order to bypass this behavior, I had to limit the scope of that first \bfseries. So I used
\patchcmd{\@makechapterhead}{\huge\bfseries \@chapapp\space \thechapter}{\huge{\bfseries \@chapapp\space \thechapter}}{}{}
(note the extra pair of curly brackets inserted to enclose \bfseries \@chapapp\space \thechapter)
After this patch then
\patchcmd{\@makechapterhead}{\Huge \bfseries}{\Huge }{}{}
works as expected.