The format of the e-mail is configured in the ccnet.exe.config or ccservice.exe.config (in C:\Program Files\CruiseControl.NET\server) if you are running ccnet a service.
I believe the file that you want to include is tests.xml as that includes a detailed listing of the nunit test results. Modify the xslFiles section of the configuration file to include this stylesheet. It will look something like this.
<!-- Specifies the stylesheets that are used to transform the build results when using the EmailPublisher -->
<xslFiles>
<file name="xsl\header.xsl"/>
<file name="xsl\compile.xsl"/>
...
<!-- add tests.xsl here... -->
<file name="xsl\tests.xsl"/>
</xslFiles>